''i'ii^^iiit!;;; 









■^Kuiiv-;;;;'!-;!!);::;?:;^^ 



mmi-tti'.r 






:i:';:H;.,:.x: 






•«s^;rjJ3] 






;iHa»ai^ 



wi? 



^^m 



'%^' 












•^v" .'f-^ 



^ -^^ 



i^^^ 



-^. .A^ 






,0 o 



■='/ " 



■>:-y^: 













\^°. 






a- 



CO 



.^■^" 



"^.- S^' 



.-^.^ 



■^ o,"i' 



,v5 



' -J^. 






c'b' 



"^^ v*^ 



.,0 



•^. e,-^'- 



.^"^ •^*. 



/ 

Ube. Science Series 

EDITED BY 

professor 5. mcTkccn CattcII, /ID.H., pb.S). 

AND 

3f. TE, 3Bc^^ar^, '^.H., df.tR.S. 




RIVERS OF NORTH 
AMERICA 



RIVERS OF NORTH 
AMERICA 

A READING LESSON FOR STUDENTS 
OF GEOGRAPHY AND GEOLOGY 



BY 



ISRAEL C. RUSSELL 

PROFESSOR OF GEOLOGY IN THE UNIVERSITY OF MICHIGAN 

AUTHOR OF "lakes OF NORTH AMERICA," "GLACIERS OF NORTH AMERICA," 

"volcanoes OF NORTH AMERICA," ETC. 



NEV/ YORK 

G. P. PUTNAM'S SONS 

LONDON 

JOHN MURRAY 

1898 



\^\^ 

e) 



18419 



Copyright, i3q8 

BY 

G. P. PUTNAM'S SONS 




2^0 ow. . 



•-^ "cocivtU. 



2ncf COPY, 
1G33. 

Zbc f^nicfccrbockcr prcas, lAcw )i?ocfe 



"Every river ap])ears to consist of a main trunk, fed from a variety of 
branches, each running in a valley proportioned to its size, and all of them 
together forming a system of valleys, communicating with one another, and 
having such a nice adjustment of their declivities, that none of them join the 
principal valley either on too high or too low a level ; a circumstance which 
would be infinitely improbable if each of these valleys were not the work of 
the streams that flow through them." — Illusti-ations of the Hiittoniaii Theory of 
the Earth : by John Flay fair. Edinburgh, 1S02, p. 102. 



TO THE READER 

EVERY person is familiar with the beating of the rain 
upon the surface of the land, and the gathering of the 
waters that fall into rills, rivulets, and brooks which fre- 
quently unite to form larger rivers. Everyone is aware, 
also, that streams are turbid after heavy rains. But, al- 
though these facts have been known to us from childhood, 
yet comparatively few people have thought out the chain of 
events of which they form a part, or recognised the results 
toward which they lead. 

Standing by the side of a river, we see its waters flowing 
continually in one direction and in many instances bearing 
along a load of sediment. We know that the mud which 
discolours the waters was derived from the lands bordering 
the stream and is journeying to the sea. So far as we can 
ordinarily discern, there is no compensation for this re- 
moval. Evidently, if the process goes on without being 
•counteracted by other agencies, all of the material forming 
land areas will in time be removed, and the hills and even 
the grandest mountains will be degraded to the level of the 
sea. We know of no reason why this process of soil removal 
may not have been in operation since rain first fell on land, 
or why it may not continue so long as continents and islands 
exist. As the land has not been reduced to sea-level, one 



VI TO THE READER 

of two conclusions seems evident : either the time that has 
elapsed since the process began has not been sufficiently 
long to bring about the final result, or else there is some 
compensating process by which land areas are renewed. 

A person who wanders along a river bank with these and 
kindred thoughts in mind naturally seeks for evidences of 
the work the rivers have accomplished and endeavours to 
learn how it has been carried on. One observes the river 
flowing through a narrow, steep-sided trench, or perhaps 
meandering in broad, graceful curves over the bottom of a 
wide, fruitful valley. In all directions the view is limited by 
hills or steeply ascending slopes. On gaining a command- 
ing station on a hilltop, the broader prospect, including 
both hills and valleys, very likely will reveal the fact that 
the hills, ridges, and more or less isolated peaks rise to the 
same general height, and appear as a level plain w^hen the 
eye, nearly on a level with their summit, ranges over them; 
and sunken in this plain is the river valley. If the river has 
been engaged for ages in carrying away the material of the 
land in the way it is now doing, the valley must represent 
a part or the whole of the work done. The thought that 
the river is older than the valley, and that the valley has 
been excavated by the river, comes to one like a revelation. 
In fancy we see the valley filled with rocks like those in the 
bordering hills and the plain restored. Evidently the sur- 
face of the region, less diversified than now, must have been 
a plain or plateau before the stream excavated its valley. 
When this idea has taken root in the mind, our powers of 
observation are stimulated and our faith strengthened in 
what has been termed the scientific use of the imagination. 



TO THE READER VI i 

As our vision ranges over valleys and hills, the fact is recog- 
nised that the neighbouring mountains are but uplands of 
larger size, separated one from another by gorges and val- 
leys, in each of which a stream is flowing, and we are 
startled by the vividness of the pictures that crowd them- 
selves on our fancy, each portraying a difl'erent stage in the 
development of a landscape which had previously charmed 
us simply by its assemblage of attractive forms and its 
harmonious blendings of colour. 

The mental pictures of the loiterer by a stream-side or 
the dreamer on a hilltop are not confined to the region im- 
mediately before him. What is true of the stream at his 
feet must also pertain in general to other streams in what- 
ever clime. Again, a curtain is lifted and he sees that every 
stream on the earth's surface is engaged in changing the as- 
pect of the land. Valleys have been excavated on every 
continent and island. Every mountain has been sculptured. 
Changes due to running water are everywhere in progress. 
If these conclusions are well founded, it is evident that 
valleys and mountains are but transient forms in a long 
process of topographical development, and the history of 
past changes should find expression in the relief of the 
land. 

The leading idea which absorbs the attention when a 
living interest is once awakened in the meaning of the many 
and diversified features of the earth's surface, is that they 
are not fixed and changeless forms, but have undergone 
many orderly modifications in the past and will continue to 
change under the action of definite laws in the future. It 
is not the shape of the earth as it exists to-day, the present 



VI li TO THE READER 

distribution of land and water on its surface, or the relief of 
the land, or of the floor of the sea, but the changes that each 
of these conditions has passed through in order to reach 
its present state, and the modifications still in progress, 
which claim the greatest share of the geographer's attention. 
Evolution is the leading theme of the inanimate as well as 
of the animate world. The features of the earth's surface, 
from the continents and oceans to the smallest islands and 
tiniest rills, all have what may be termed their life histories. 
It is the recognition of this fact that has given new interest 
and imparted a fresh impetus to geographical study. 

When once the idea is grasped that each and every one of 
the elements in a landscape has a history w^hich can be read, 
and that the end is not yet, but still other transformations 
are to come, an insatiable desire is awakened for more 
knowledge concerning especially the work of the streams to 
which so many of the changes that have been made on the 
earth's surface are due. What laws do the streams obey ? 
What conditions modify their normal behaviour ? What 
are the various stages in the transformations they are making 
everywhere about us ? Are there other forces in action, 
tending to counteract their destructive work ? Are the 
conditions to which man has become adjusted to pass away ? 
What changes are to come ? Is the re-modelling of the 
land to be continued for ever, or will there be a final condi- 
tion beyond which the expression of the face of Nature will 
become sphinx-like and unchanging ? These and many 
more queries crowd themselves on the mind when once an 
interest in the daily scenes about us is awakened. Many, 
but by no means all, of the questions which we wish to ask 



TO THE READER IX 

of the mountains and streams can be satisfactorily answered 
at the present day. 

It is with the hope of assisting the reader both in ques- 
tioning the streams and in understanding their answers, and 
at the same time creating a desire for more light on other 
and related chapters of the earth's history, that the book 
before you was written. 

The study of the earth's surface should be of especial in- 
terest to American students not only because of the mag- 
nificent and varied scenery of our native land, but for the 
reason that new life and vividness have been given the sub- 
ject by the labours of men who are still among us. 

The marked advances made during the present century 
both in the study of the ancient life on the earth and of the 
surface changes still in progress, show the influence of en- 
vironment. The Geological Survey of the State of New 
York gave a marked impetus to the study of the invertebrate 
life of distant ages, largely for the reason that the rocks of 
the New York series are rich in such relics. When geolo- 
gists visited the central and western portions of the United 
States, the sediments of ancient lakes, rich in vertebrate 
fossils, were discovered. The veritable menagerie of mar- 
vellous birds, reptiles, and mammals that has been made to 
appear from these cemeteries is more varied than the most 
fantastic dreams of fable. We feel that the uncouth pro- 
cession has only begun to pass in review, but are at a loss 
to imagine what as yet unknown commingling of fish, rep- 
tile, bird, and mammal in one and the same individual can 
possibly be found. When investigators of surface geology 
and geography made their bold explorations into the vast 



X TO THE READER 

arid region of the south-west, they discovered a land of 
wonders, where the mask of vegetation which conceals so 
many countries is absent, and the features of the naked land 
are fully revealed beneath a cloudless sky. The facts in 
the earth's history which there impress themselves most 
forcibly on the beholder are such as have resulted from the 
action of streams and of atmospheric agencies. It was in 
this arid region of strong relief that a revival of interest in 
the surface forms of the earth was engendered. The seeds 
of what is practically a new science, — physiography, — 
gathered in this desert land by J. S. Newberry, J. W. 
Powell, G. K. Gilbert, C. E. Button, and others, when 
carried to other regions bore abundant fruit. It was found 
that the surface of the land, when once suggestion had en- 
abled men to see topographic forms and interpret their 
meaning, is a manuscript on which wonderful events are 
recorded. 

A younger generation of active workers has extended the 
study of the earth's surface, so greatly stimulated by the 
pioneer explorers just named, and has read for us the his- 
tories of valleys, plains, hills, and mountains throughout the 
length and breadth of the land. Of these younger investi- 
gators we are indebted to none so much as to W. M. Davis, 
Professor of Physical Geography at Harvard, who has made 
New England, New Jersey, and Pennsylvania classic ground 
to all future students of physiography. Gilbert has ex- 
tended his studies to the basins of the Laurentian lakes and 
other regions. The writings of J. C. Branner, IM. R. Camp- 
bell, T. C. Chamberlin, N. H. Darton, J. S. Diller, C. W. 
Hayes, Arthur Keith, W J McGce, R. D. Salisbury, R. 



TO THE READER XI 

S. Tarr, Bailey Willis, and others, have greatly enlarged our 
knowledge of the laws governing streams, and of the origin 
of topographic forms. 

The publications of the United States Geological Survey 
and of the earlier national surveys of which it is a continua- 
tion, the Geological and Natural History Survey of Canada, 
various State surveys, the Geological Society of America, and 
the National Geographic Society, together with the Journal 
of Geology^ the American Journal of Science j etc., are the 
sources of published information drawn on most largely in 
the preparation of this book. 

In the following chapters many references are given to 
the writings of the distinguished investigators named above, 
but all of the help derived from them while writing this 
book can scarcely be acknowledged. Much assistance has 
also been derived from conversation and correspondence 
with my colleagues, and, although fully appreciated, the 
portion derived from each one is scarcely known even to 
myself. My part in presenting this book is largely that of 
a guide who points out the routes others have traversed. 
My reward will be ample if even a few students follow the 
paths indicated and are led to explore their many as yet 
unknown branches. 

Israel C. Russell. 

University of Michigan, 
December lo, 1897. 



CONTENTS 



PAGE 

To THE Reader v 



CHAPTER I 

The Disintegration and Decay of Rocks 

Mechanical Disintegration — Chemical Disintegration or Rock Decay 
— Removal and Renewal of Surface Debris. 



CHAPTER n 
Laws Governing the Streams ........ 12 

How Streams Obtain their Loads — Transportation — Debris Carried 
by Ice — Corrasion — Pot- Holes — Lateral Corrasion — Meandering 
Streams — Other Curves — Deflection of Streams Owing to the Earth's 
Rotation — Questioning the Rivers — Erosion — Baselevel of Erosion — 
Peneplains — Influence of Vegetation on Erosion. 

CHAPTER III 

Influence of Inequalities in the Hardness of Rocks on River- 
side Scenery 52 

Waterfalls — The Migration of Waterfalls — Bluffs Bordering Aged 
Streams. 

CHAPTER IV 

Material Carried by Streams in Suspension and in Solution 67 

The Visible Loads of Streams — Bottom Load — Measures of Material 
in Suspension — The Invisible Loads of Streams — Rate of Land 
Degradation — Mechanical Degradation — Chemical Degradation — 
Rate of Both Mechanical and Chemical Degradation — Underground 
Streams. 



XIV CONTENTS 

CHAPTER V 

FACE 

Stream Deposits 97 

Alluvial Cones — Talus Slopes — Flood-Plains — Natural Levees — 
Deltas : Deltas of High-Grade Streams — Deltas of Low-Grade 
Streams — Effects of Changes in the Elevation of the Land on 
the Growth of Deltas — Variations in Normal Stream Deposition — 
Influence of Elevation and Depression of the Land on Stream 
Deposition — Influence of Variations in Load on Stream Deposition 
— Influence of Changes of Climate on Stream Deposition — The 
General Process of Stream Corrasion and Deposition — Profiles of 
Streams — The Longitudinal Profile — Cross-Profiles. 



CHAPTER VI 

Stream Terraces 

Origin of Terraces during the Process of Normal Stream Develop- 
ment — Terraces Due to Climatic Changes — Terraces Due to Eleva- 
tion of the Land — Bottom Terraces — Delta Terraces and Current 
Terraces — Glacial Terraces — Relative Age of Terraces — Other Ter- 
races — General Distribution of Stream Terraces. 



CHAPTER VII 

Stream Development 184 

Consequent Streams — Subsequent Streams — Ideal Illustration of 
Stream Adjustment and Development — Examples of Stream Develop- 
ment and Adjustment in the Appalachian Mountains — Influence of 
Folds in the Rocks on Stream Adjustment — Water-Gaps and Wind- 
Gaps — Stream Conquest — Ancient Peneplains — Synclinal Mountains 
and Anticlinal Valleys — Effects of Elevation and Subsidence on 
Stream Development — Some of the Effects of Elevation — Some of 
the Effects of Subsidence — Some of the Influences of Volcanic 
Agencies on Stream Development— Some of the Modifications in 
Stream Development Due to Climatic Changes — Variations in Pre- 
cipitation — Variations in Temperature — Fluctuations of Streams — 
Some of the Influences of Glaciers on Stream Development — Some 
of the Influences of Vegetation on Stream Development — Driftwood 
— Superimposed Streams — Migration of Divides. 



CONTENTS XV 

CHAPTER VIII 

PAGE 

Some OF THE Characteris lies OF American Rivers . . . 254 

Drainage Slopes : Atlantic, St. Lawrence, Hudson Bay, Arctic, 
Bering, Pacific, Great Basin, Gulf, and Caribbean — Leading Features 
of the Several Drainage Slopes — New England Rivers — A Drowned 
River — Appalachian Rivers — Rivers of Glaciated Lands — Southern 
Rivers — Alluvial Rivers — The Mississippi — Canyon Rivers — Sierra 
Nevada Rivers — "Where Rolls the Oregon" — Rivers of the Far 
North-West — Glacier-Born Rivers — Arctic Rivers — Rivers of the 
*' Great Lone Land" — Rivers Flowing to Fresh- Water Seas — 
Niagara — Retrospect. 

CHAPTER IX 

The Life History of a River ........ 301 

Index 321 



ILLUSTRATIONS IN THE TEXT 



FIGURE 

1. A POT-HOLE BEING SCOURED OUT BY A STREAM 

2. A Young Stream near Ithaca, New York 

3. Profile of Niagara Falls 

4. Cross-Profile of a Floodplain .... 

5. Map of the Lower Mississippi showing Crevasses 

6. Radial Section of a Delta .... 

7. Longitudinal Profile of a Young Stream 

8. Successive Changes in the Profile of a Divide 

9. Ideal Profile of a Divide 

10. Cross-Profile of a Terraced Valley 

11. Alluvial Terraces 

12. Alluvial Terraces 

13. Cross-Section of a Valley with Terraces in Solid 

14. Cross-Section of a Current-built Terrace 

15. Section of tilted Peneplain .... 

16. Sketch-Map, showing Young Streams 

17. Sketch-Map, illustrating Stream Development 

18. Sketch-Map, showing Mature Streams . 
ig. Anticlinal and Synclinal . 

20. Map illustrating River Piracy 

21. Section through Lookout Mountain 

22. Map of Chesapeake Bay 

23. Cross-Profile of Colorado Canyon 
Table A. Analysis of American River-Waters 



etc., Alabama 



Rock 



facing 



PAGE 

33 

55 

60 

118 

119 

126 

147 
148 
149 
152 
156 
157 
165 
168 
186 

187 
189 
190 
198 
200 
212 
219 
272 
78 



FULL-PAGE ILLUSTRATIONS 

PLATE FACING PAGE 

I. a. Marion River, New York. b. Ingall's Ceeek, Wash- 
ington 12 

II. Views on the Yukon, Alaska 24 

III. a. Ray Brook, Adirondacks, New York. b. Moccasin 

Bend, Tennessee River 38 

IV. a. Fall on Black Creek, near Gadsden, Alabama, b. 

Echo River, Mammoth Cave, Kenjucky ... 60 

V. Map of the Delta of the Mississippi 98 

VI. a. Sketch of Alluvial Cones, b. Indian Creek, Cali- 
fornia 102 

VII. a. Big Goose River, Wyoming, b. New River, Tennessee, 108 
VIII. a. Terraces on Fraskr River, British Columbia, b. Ter- 
races in Connecticut Valley . . . . . 154 

IX. Map of the Northern Appalachians 196 

X. Map of Western Portion of the Anthracite Basin, 

Pennsylvania 204 

XI. Map illustrating Stream Adjustment . . . .210 
XII. a. Beaver Dam, Wyoming, b. Dam of Drift-wood, West 

Fork of Teanaway River, Washington ... 238 

XIII. Map of a Portion of the Catskill Moun-^ains, New York, 250 

XIV. Map of North America showing Drainage Slopes . . 256 
XV. a. Columbia River, b. Hudson River .... 262 

XVI. a. An aggraded Valley near Fort Wingate, New 
Mexico, b. Shenandoah Peneplain, near Harper's 

Ferry, West Virginia 26S 

XVII. Canyon of the Colorado 274 



RIVERS OF NORTH AMERICA 



CHAPTER 1 

THE DISINTEGRA TION AND DEC A Y OF ROCKS 

THE study of rivers, from the point of view of the geo- 
grapher, necessitates the consideration of the nature 
and origin of many topographic forms; the reason being 
that streams are among the most important agencies which 
give form and expression to the surface of the land. The 
study of streams, therefore, involves, to a great extent, the 
consideration of the origin of hills and mountains, plains 
and valleys, and the changes they pass through. 

One of the principal tasks performed by streams is the 
moving of rock fragments and their transportation to the 
sea. Another function of streams is the deepening and 
widening of their channels and valleys. These propositions 
will be demonstrated later. It will be shown, also, that 
clear water has but little power to wear away the rocks 
over which it flows. In order to do this, the flowing water 
must be charged with hard particles or rock fragments of 
greater or less size. That is, the streams must be supplied 
with tools with which to excavate. It is now well under- 



2 RIVERS OF NORTH AMERICA 

stood that the tools used by streams in abrading the rocks 
are mainly silt, sand, gravel, and stones, which are carried 
in suspension or rolled and pushed along the bottom. One 
of the primary questions, therefore, in order to understand 
how streams are enabled to remove material from the land, 
and, in so doing, to deepen and broaden their valleys, is: 
How are the rocks broken or otherwise prepared for stream 
transportation ? 

The soil which nearly everywhere forms the surface of the 
land is composed mainly of disintegrated rock. This loose 
surface layer of more or less comminuted and decayed 
material, much of it, however, far too coarse to be termed 
soil, is the storehouse from which the streams derive the 
principal part of their loads. 

The study of the agencies at work in breaking and other- 
wise disintegrating the earth's crust has shown that they 
may be classified in two groups: 1st, those acting mechani- 
cally; and 2d, those whose influence is principally chemical. 
Although the various agencies in these two groups co- 
operate and are frequently in action at the same time, it is 
convenient to consider them separately. 

Mechanical Disintegration. — Changes of temperature, as 
between day and night, or from season to season, cause un- 
equal expansion and contraction of the minerals and grains 
of which rocks are composed. Various and complex stresses 
are thus produced which cause even the most compact 
granite to crumble. The freezing of water contained in 
crevasses in rocks or in the interspaces between grains or 
crystals, is accompanied by expansion, which exerts a 
powerful force tending to fracture and disintegrate them. 



DISINTEGRATION AND DECAY OF ROCKS 3 

The roots of trees enter crevices in the rocks, and as they 
enlarge, force off fragments frequently of large size. The 
undercutting of cliffs and banks by streams and by the 
waves and currents of lakes and of the ocean, causes the dis- 
lodgment of vast quantities of earth and stone. The fall of 
rocky material produced by these and still other causes, 
leads to still further breakage. Rock masses are also loos- 
ened or caused to fall by earthquake shocks. Volcanoes 
discharge a great volume of fragmental material into the 
air, and the cooling of lavas causes them to become frac- 
tured and jointed. When molten lava enters surface-water 
bodies, steam and gas explosions occur, and the rock is 
perhaps blown to dust. Rain-drops, snow crystals, and 
hail by beating on the rocks exert a force tending to break 
off fragments loosened by other and principally chemical 
agencies, and to wear, and frequently to polish, exposed 
surfaces. Sand and dust, blown by the wind, on coming in 
contact with rock exposures, wear away the softer parts and 
loosen the harder grains and crystals. Glaciers as they flow 
down mountain valleys or move over the surface of more 
level land, tear away projecting ledges, and when charged 
with sand and stones abrade and grind away the rock over 
which they move. Avalanches and landslides rush down 
declivities, carrying destruction in their paths, and sweep 
along loosened rock fragments which are broken still finer 
and in part ground to powder. The streams themselves, 
under certain conditions, roll along stones and even large 
boulders, which become rounded and broken, at the same 
time abrading the rocks over which they are carried, and 
thus aid in the general process of rock disintegration which 



4 RIVERS OF NORTH AMERICA 

prepares the material composing the land for stream trans- 
portation. 

All of the agencies just enumerated are mechanical in 
their action, although accompanied by chemical changes, 
and are confined to the surface, or, at most, to an extremely 
superficial portion, of the earth's crust. There are also im- 
portant mechanical agencies which act deep below the sur- 
face and lead to the fracturing of the rocks in such manner 
as greatly to facilitate the agencies producing disintegration 
in operation at the surface. And, besides, on account of 
the continual lowering of the surface in many regions owing 
to the removal of material, rock fragments originating at a 
greater or less depth become mingled with those produced 
at the surface in the several ways just enumerated, and thus 
become of interest in the study of the manner in which 
rocks are reduced to fragments of such size that they can 
be moved by streams. 

Of the mechanical agencies leading to the fracturing of 
rocks below the reach of frost and of normal changes in 
temperature, the most important are movements in the 
earth's crust, the nature of which it is impracticable to dis- 
cuss at this time, which cause even the most massive layers 
to become folded and broken. These movements are fre- 
quently accompanied by the crushing of the rocks in zones 
of various widths, as when a fracture is formed and its walls 
ground against each other. Such breaks, accompanied or 
followed by differential movements of their wails, are 
termed faults, and the rock fragments produced are desig- 
nated fault breccias. 

The world over and to a great but indefinite depth, the 



DISINTEGRATION AND DECAY OF ROCKS 5 

rocks are divided by what are known as joints, the origin of 
which is obscure. These dividing planes are similar, we 
may fancy, to gashes made by a sharp blade without appre- 
ciable thickness, drawn through the rocks. There are fre- 
quently two series of joints nearly at right angles to each 
other, and more or less nearly vertical ; these are intersected 
many times by approximately horizontal cuts of the same 
character, and frequently also by planes of bedding. The 
rocks are divided in this manner into masses that are some- 
times nearly true cubes. In many instances the joints cross 
each other irregularly and divide the rocks into blocks of 
many shapes. Joint blocks of whatever form vary in size, 
from a small fraction of a cubic inch to many cubic feet. In 
some instances these are contributed directly to streams, as 
when they fall from the face of a precipice, but more com- 
monly they are broken and variously modified by atmos- 
pheric agencies before being fed to the flowing waters. 
The joints in rocks, although of inappreciable width deep 
below the surface, are planes of weakness along which 
chemical and mechanical agencies find favourable lines of 
attack. They open when the rocks are exposed to the 
weather, and greatly favour the further disintegration re- 
sulting from changes of temperature, the freezing of water, 
etc. The jointing of rocks is one of the primary and most 
important methods by which they become divided into 
blocks, thus exposing greater surfaces to the attack of 
chemical agencies, and in many regions, particularly in 
rugged mountains and in canyon walls, exerts a direct and 
pronounced influence on topographic forms. 

Among the agencies that lead to the fracturing and me- 



6 RIVERS OF NORTH AMERICA 

chanical disintegration of rocks deep below the surface, 
should be noted, also, injections of molten rocks forced up- 
ward into the earth's crust, earthquake shocks, the friction 
of debris carried by subterranean streams, and the falling of 
cavern roofs. Still another agency, as has been pointed out 
by G. P. Merrill, in part chemical and in part mechanical in 
its action, results from the combination of water with certain 
mineral substances, producing what is termed hydration. 
This is accompanied by an increase in the bulk of the min- 
erals affected, and the consequent production of stresses in 
the rocks containing them. In some instances, apparently 
unaltered rock, when removed from mines and tunnels, 
rapidly crumbles from this cause when exposed to the air. 
In nature, the lowering of the surface by erosion and the 
exposure of previously deeply buried rocks would bring 
about similar changes. There are yet other alterations in 
progress in the rocks due to chemical action, that promote 
mechanical disintegration, which cannot be noted at this time. 

By the several processes just enumerated, the rocks are 
broken into blocks of all shapes and dimensions, some of 
which are of the size of gravel, sand, and dust grains, and 
are thus rendered. suitable for stream transportation, and to 
act as tools by means of which flowing water promotes the 
process of rock breakage. 

Chemical Disintegration or Rock-Decay, — Water is a solv- 
ent for probably all substances that occur in the earth's 
crust, although in many instances acting with extreme slow- 
ness. The readiness with which most substances are taken 
into solution by water is enhanced by an increase of tem- 
perature, and in nature is also greatly assisted by various 



DISINTEGRA TION AND DEC A V OF ROCKS 7 

substances, especially organic acids, with which it becomes 
charged. 

Even rain-water is never pure, but contains various salts 
and gases derived from the air. Principal among these is 
carbon dioxide, or carbonic acid. Rain-water on reaching 
the earth flows over the surface, or percolates for a time 
through the soil and rocks, and thus comes into intimate re- 
lations with the great store of organic acids supplied by the 
waste and decay of animal and vegetable life. The chemi- 
cal energy of the water is thus greatly enhanced, and it 
becomes an active solvent for most mineral substances. 
Some of the minerals composing rocks are more soluble 
than others, and, being removed, allow those that remain 
to crumble and fall apart. The mineral substances taken in 
solution are, for the most part, contributed to streams and 
by them carried to the sea as an invisible load ; but a por- 
tion is taken below the surface by downward-percolating 
waters and undergoes many changes in composition and at 
the same time produces various alterations in the rocks 
through which it passes. 

The chemical changes produced in the rocks by percolat- 
ing waters, while most active near the surface, occur also at 
considerable depths, and are there augmented by the inter- 
nal heat of the earth. There is a lower limit to this process, 
however, due to the increasing density of the rocks with 
pressure and to the rise of temperature with increase in 
depth. There are good reasons for concluding that surface 
waters cannot descend more than twenty thousand or 
thirty thousand feet below the surface. 

The chemical changes due to percolating water are influ- 



8 RIVERS OF NORTH AMERICA 

enced in a variety of ways by temperature. The rocks are 
dissolved most readily in warm, moist regions. It is in such 
regions also that vegetation is most luxuriant and animal 
life most abundant, and hence the waters are most highly 
charged with organic acids. Chemical action, in most in- 
stances, is retarded by cold; vegetation is less abundant 
and decay less rapid in cold than in warm climates; it is, 
therefore, in cold regions that the decay of the rocks is at 
a minimum. 

In warm, humid countries, deep rock-decay has usually 
taken place, but a thick surface sheet of decomposed ma- 
terial is not necessarily found, as the loosened debris may 
be carried away as fast as it is produced. In the southern 
Appalachians, and in many other warm temperate or equa- 
torial regions, the rocks are so broken and decayed, even at 
a depth of one hundred and fifty or two hundred feet from 
the surface, that they may be crumbled between the fingers 
or moulded like clay. In such instances the soil usually 
shows various tints of red and yellow, the colours being due 
to the oxidation and hydration of iron present in them. 

In warm, dry countries chemical changes in the surface 
material are retarded, although the rocks may be greatly 
shattered by changes of temperature. In such regions the 
soils are seldom red. 

Chemical changes produced by percolating water below 
the superficial portion of the earth — that is, below, per- 
haps, one hundred feet — increase with depth, on account of 
progressively increasing temperature, but these changes 
are beyond the immediate subject under discussion. An 
important agency in rock disintegration and decay having 



DISINTEGRA TION AND DEC A V OF ROCKS g 

its source deep within the earth, however, is manifest espe- 
cially in volcanic regions where steam charged with various 
acids rises through fissures and other openings. During 
volcanic eruptions, but more particularly after a volcano has 
passed to the condition of a fumarole or a solfatara, heated 
vapour and gases charged with sulphuric, hydrochloric, car- 
bonic, and other acids escape in large volumes, sometimes 
continuously for centuries, and produce conspicuous changes 
in the rocks through which they rise. Similar but usually 
less copious exhalations occur from lava streams, and pro- 
duce alterations in the lava which influence the character of 
the soil resulting from them. 

The chemical alterations produced by percolating water, 
and less commonly by volcanic gases, in rocks near the 
surface, are in part by solution, and in part oxidation, 
hydration, precipitation, etc. These changes, except the 
last mentioned, may be grouped, at least in a general way, 
under the term rock-decay. In decaying, the rocks are 
more or less disintegrated, however, since the more soluble 
minerals are removed, thus allowing the less soluble rock 
constituents to crumble and fall apart. The processes of 
rock-disintegration and rock-decay mutually assist each 
other, and progress at the same time. The result is that 
the surface layer of the earth's crust is profoundly altered, 
and a sheet of modified material is produced, which is 
designated in part as soil and in part as rock detritus.* 

Tlie Removal and Renewal of Surface Debris. — The sur- 

' The name Regolith, meaning blanket-stone, has recently been proposed for 
the superficial material covering the earth, by G. P. Merrill, A Treatise oh 
Rocks, Rock- Weathering and Soils^ 1897, p. 299. 



lO RIVERS OF NORTH AMERICA 

face changes just considered have been in progress since the 
first appearance of land, and will continue as long as conti- 
nents and islands exist. In former geological periods the 
agencies enumerated, particularly those of a chemical nature, 
were more active than now, and have varied from time to 
time, in probably all portions of the earth's surface, with 
climatic and other changes. Throughout all geological ages, 
the streams have been actively engaged in removing the dis- 
integrated and more or less chemically altered surface por- 
tions of the earth's crust. In places and at certain times, 
the debris has been removed as fast as formed, and bare, 
hard rock surfaces have been exposed ; at other times, the 
supply has been in excess of the demand, and deep accumu- 
lations have resulted. The surface sheet of debris has been 
continually wasting and continually renewed. Throughout 
the history of the earth, topographic changes have been in 
progress. Mountain ranges and systems have been upraised, 
the rocks composing them fractured and chemically altered, 
and borne away by streams. Where once a magnificent 
mountain range reared its battlements among the clouds, 
there is now a plain but little elevated above the sea. Not 
only one, but several such geographical cycles have run their 
courses in many lands. 

During the cycles still in progress human agencies have 
been added to those previously in action. This new element 
in the earth's history has become more and more important 
as man has advanced in civilisation. In part, human indus- 
tries have retarded the work of physical and chemical agen- 
cies, but in the main man has been a destroyer. 

The removal of the portion of the earth's crust rising 



DISINTEGRA TION AND DECA V OF ROCKS 1 1 

above the sea, during each cycle, has been done almost 
wholly by streams. The manner in which the rocks are 
prepared for transportation, however, is quite as important 
to the geographer as the methods employed for their re- 
moval, but the brief review given above of this division of 
the general process must suffice for the present. 

The student who may wish to continue the studies out- 
lined in this chapter will find assistance in the books men- 
tioned below. ^ 

^ George P. Merrill. A Treatise on Rocks, Rock- Weathering and Soils, 
pp. 172-398. The Macmillan Co., 1897. 

Israel C. Russell. The Decay of Rocks and the Origin of the Red Colour 
cf Certain Formations. U. S. Geological Survey, Bulletin No. 52, 1889. 

Israel C. Russell. "A Reconnoissance in South-Eastern Washington." 
U. S. Geological Survey, Water-Supply and Irrigation Papers, No. 4, pp. 57- 
69, 1897. 

George P. Marsh. The Earth as Modified by Human Action. Charles 
Scribner's Sons, 1885. 

John C. Branner. '* Decomposition of Rocks in Brazil." Bulletin of the 
Geological Society of America, vol. vii., pp. 255-314, 1896. 

Alexis A. Julien. "On the Geological Action of the Humus Acids," in 
American Association for the Advancement of Science, Proceedings, vol. xxviii., 
pp. 311-410, 1879. 

Walter Maxwell. Lavas and Soils of the Hawaiian Islands. Honolulu, 
i8q8. 



CHAPTER II 

ZAIVS GOVERNING THE STREAMS 

THE water which flows off from the land, as is well 
known, is supplied by the condensation of vapour in 
the air. A part of the water reaching the earth flows over 
the surface and gathers into rills which unite to form larger 
streams, and a part sinks below the surface and, after follow- 
ing an underground course, usually by percolation through 
porous soil or rocks, emerges in springs, many of which join 
the surface flow. 

It is also well known that streams ranging in size from the 
smallest rills to the mightiest rivers are engaged either oc- 
casionally, as during floods, or continually, in carrying away 
material that was previously a portion of the land. The 
manner in which this material is acquired by the streams, 
the way it is transported, the effects it has on the flow of 
the streams, and on their bottoms and sides, the modifica- 
tions in the configuration of the surface of the land due to 
the removal and re-deposition of debris, etc., are all phe- 
nomena that obey definite laws and are variously modified 
by conditions. If one can ascertain the laws governing the 
behaviour of a single stream, they should also apply not 
only to the streams of North America, but to those of all 
land areas. 

12 



i 



Plate I. 




Fig. a. Marion River, Adirondacks, New York. 

Summer stage, showing stones which are moved during high-water. (Photograph by 
S. R. Stoddard.) 




Fig. B. Ingall's Creek, Washington. 
Showing boulders too large for the stream to move even during high-water stages. 



LAWS GOVERNING THE STREAMS 1 3 

No one stream, perhaps, in the limited time that an in- 
dividual student is enabled to examine it, will furnish illus- 
trations of all of the modifying conditions influencing the 
life of a great river. By selecting typical examples, how- 
ever, affected by different modifying conditions, we may 
sketch a composite picture which will represent the various 
phases in the life history of a single river that has carried on 
its work for tens of thousands of years. 

How Streams Obtain their Loads, — Rain-drops strike the 
earth with a certain force, dependent on their size, the dis- 
tance they descend, the direction and force of the wind, 
etc. If rain-drops fall on the surface of a still pool we may 
see them rebound. If we face a rain-storm, the sting of the 
beating drops again assures us that they exert a consider- 
able force on the objects against which they strike. When 
the drops fall on a solid rock surface, they gather in rills of 
clear water, but if they fall on loose soil, as a newly 
ploughed field, for example, the finer particles of earth are 
disturbed, and as the waters gather into rills and flow away 
in obedience to gravity, they are turbid with earth particles 
held in suspension. The turbid rills unite in brooks, and 
these again combine to form larger streams. The fine silt 
disturbed by the impact of the rain-drops is carried by the 
rills to the brooks and thence onward, perhaps with many 
halts, to the sea. After heavy rains even large rivers be- 
come muddy. The lakes and large areas in the sea near 
the mouths of rivers are then discoloured. This happens 
during every storm the world over, and evidently, if suffi- 
cient time be allowed, must lead to changes of great mag- 
nitude both in the topography of the land from which 



14 RIVERS OF NORTH AMERICA 

material is removed and in the shape of the basins where it 
is deposited. 

When the surface of the land is dry, and especially when 
bare of vegetation, earth particles are moved by the wind in 
much the same manner that streams take up and transport 
the flakes and grains of rock which they are competent to 
transport. The dust and sand carried by the wind and 
falling in streams is another source from which they obtain 
material suitable for removal. 

The fine rock powder, or glacial meal as it is termed, 
produced by the grinding of stones held in the ice against 
each other and on the rocks over which the glaciers flow, is 
contributed directly to the waters formed by the melting of 
the ice. For this reason nearly every glacier-born stream is 
turbid and heavy with silt. 

Volcanoes during times of violent eruption discharge vast 
quantities of fine dust, and in many instances equally abund- 
ant rock fragments of the size of sand and gravel. When 
material extruded in these forms falls in streams, or is car- 
ried in by tributary rills and by the wind, another source is fur- 
nished from which streams receive their initial loads. There 
are yet other methods by which streams are supplied with 
material in a suitable condition to be transported. Among 
these may be noted the fall of cosmic dust, disturbances 
produced by avalanches and landslides, the uprooting of 
trees, the impact of driftwood and floating ice on the bot- 
toms and sides of stream channels, the movement of roots 
and overhanging branches by the wind and by the currents 
of the streams themselves, disturbances of the material 
forming the bottoms and sides of stream channels by ani- 



LAWS GOVERNING THE STREAMS I 5 

mals, — as by beavers, for example, — contributions of shells 
and siliceous cases from organisms like mollusks and diatoms 
living in the waters, etc. Man promotes the transfer of 
solid matter to the streams in various ways, more especially 
by ploughing and otherwise disturbing the soil, and by the 
removal of forests. All of the methods mentioned in this 
paragraph, however, are of secondary importance in com- 
parison with the influence of rain, wind, and glaciers. 

The particles carried in suspension by streams tend to 
fall to the bottom, being continually pulled down by grav- 
ity, but in flowing water there are various currents, some of 
which tend upward and exert an influence on the falling 
particles in opposition to gravity. The currents in the 
water move the suspended particles in various directions 
and retard their fall to the bottom, but the resultant move- 
ment is in the direction of the flow of the stream. 

The particles carried by streams fall to the bottom many 
times during their journeys, and rest there for a period per- 
haps brief but possibly long, and are again lifted by upward 
currents and brought within the influence of the onward 
flow. Material thus transported by a stream may for con- 
venience be termed its load. Streams not only receive their 
initial loads in the various ways just stated, but there are 
other methods by which the same result is reached. As 
will be considered later, flowing water exerts a pressure on 
objects against which it strikes. If this force be great 
enough to move the objects in the path of a stream, they 
will be pushed along, rolled over, or, with the assistance of 
upward currents, taken in suspension. The strength of the 
water current determines the size of the particles it can 



1 6 RIVERS OF X OR Til AMERICA 

carry, so that a stream of a given velocity, but without an 
initial load, would be able to remove from its channel all of 
the loose particles it is competent to carry and thereafter 
would run clear unless its velocity were increased. 

The principal methods by which streams receive their 
initial loads insure a waste of the land between the drain- 
age lines, and consequently this land changes in topographic 
form. The deepening and broadening of the stream chan- 
nels is accomplished principally by the friction of the debris 
carried through them, aided also by solution. The laws 
governing this complex process will be considered later. 

Transportation, — The debris acquired by streams in the 
several ways considered above is carried along by them. A 
convenient term for this process is transportation. Light 
objects like leaves, wood, pumice, etc., are floated on the 
surface, but meet with various delays, and undergo more or 
less chemical and mechanical changes during their journeys, 
and sooner or later sink to the bottom. Material like fine 
sand and silt remains in suspension to a great extent and is 
carried bodily onward. Heavier objects, like pebbles and 
boulders, are either rolled or pushed along the bottom, or 
remain at rest until they are reduced in size by the friction 
and solution, or shattered by the impact, of material swept 
against them. The size, weight (specific gravity), and form 
of the loose material within the influence of a stream deter- 
mine whether or not it will be moved by a current of a 
given velocity. The smaller the divisions into which a mass 
of rock is broken, the larger the ratio of surface to weight. 
The force which a current of a given velocity exerts against 
objects in its path varies as the area of the opposing sur- 



LAJVS GOVERNING THE STREAMS 1 7 

face. The smaller the parts into which a rock mass be- 
comes divided, therefore, the greater the tendency of the 
current to move them. 

The ability of the stream to carry debris in suspension, 
however, depends not only on its velocity and the degree of 
comminution of the material within its influence, but also, 
as previously stated, on the presence of secondary and es- 
pecially of upward currents which tend to lift the particles i 
brought within their influence. While a particle is in sus- 
pension the onward currents bear it along, but gravity is all 
the while acting, and, unless counteracted, finally pulls it 
to the bottom. The journey of a rock fragment from the 
mountains to the sea consists of a great number of upward 
and onward excursions, with rests of greater or less length 
between. More definitely, the ability of a stream to hold 
debris in suspension is due to the fact that different layers 
of water are actuated by different velocities, and these exert 
different pressures upon the different sides of the suspended 
particles. Hence, the greater the differences in the veloci- 
ties of consecutive layers, the greater will be the tendency 
to hold material in suspension. It is stated by Humphreys/ 
and Abbot, from whose report on the Mississippi much ofl 
this discussion of the mechanics of stream flow is taken, 
that the change in the velocity of the waters of streams in 
horizontal planes is greatest near the shore and least near 
the thread of maximum current; and in vertical planes, is 
greatest near the bottom and surface and least at about one- 
third of the depth of the stream — that is, where the abso- 
lute velocity is greatest. If, then, the water be either 
charged to its maximum capacity or overcharged with sedi- 



1 8 RIVERS OF NORTH AMERICA 

Iment, the highest percentage of material in suspension will 
be found near the banks and near the surface and bottom, 
and the least amount near the thread of the maximum cur- 
rent and at a depth of about one-third of that of the stream. 
If, however, the water is undercharged with material in 
suspension, the distribution will not follow any law, the 
amount at any locality being determined by what may be 
.considered as accidental swirls, boils, etc. As most streams 
are undercharged, it follows that samples of water from 
several points in a cross-section should be examined in 
order to ascertain. approximately the amount of material 
that is being carried. Rock fragments too heavy to be 
lifted may be rolled or pushed along the bottom, or perhaps 
turned over from time to time by the resultant onward cur- 
rent. There is thus an adjustment between the strength of 
the current and the specific gravity of the material trans- 
ported. 

It is well known that the power flowing water has to trans- 
port rock debris increases with increase of velocity. Experi- 
ments have shown that if water is made to flow through an 
even channel and the rate of flow is gradually increased by 
increasing the inclination of the channel, it will move ma- 
terial added to it approximately as follows ^ : 



VELOCITY OF CURRENT. 


SIZE OF MATERIAL MOVED. 


3 inches per second. 


Fine clay and silt. 


6 


Fine sand. 


12 '* " " 


Pebbles \ inch in diameter. 


2 feet 


ii I *' *' 



^ David Stevenson, Canal and River Engineering, p. 315 ; A. J. Jukes- 
Browne, Physical Geology, 1892, p. 130 ; Archibald Geikie, Text-Book of 
Geology, 2d edition, 1885, p. 354 ; Joseph Le Conte, Elements of Geology y. 
4th edition, 1896, pp. 18-20. See also other elementary works on geology. 



LAWS GOVERNING THE STREAMS 



19 



VELOCITY OF CURRENT. 


SIZE 


OF 


MATERIAL MOVED. 


2.82 


feet 


per second. 


Pebbles 2 inches in diameter. 


3.46 










3 " 


4 










4 - 


4.47 










5 " 


4.90 










6 " 


5.29 










7 " 


5.65 










8 " 


6 










9 " 



It must be understood that the currents referred to in 
this table are bottom currents, and in general may be taken 
at about one-half the central surface current. 

An important fact shown by these and other sirnilar ex- 
periments is that the transporting power of running water 
increases in a greater ratio than the increase in velocity. 

It has been demonstrated that if the surface of an object 
opposed to a current of water, as the pier of a bridge, for 
example, remains constant, the force of current striking it 
varies as the square of its velocity. Also, that the trans- 
porting ^^ow^x of a current, or the weight of the largest frag- 1 
ment it can carry, varies as the sixth power of the velocity,^ 
Under this law it will be seen that doubling the velocity of 
a current increases its transporting power sixty-four times. 
If a stream flowing with a given velocity is able to move 
stones weighing one pound, by doubling the velocity 
boulders weighing sixty-four pounds can be carried ; and if 
the velocity were increased ten times, rocks weighing one mil- 
lion pounds could be moved. This enables us to see how 
streams are capable of producing such striking results during 
floods, when their velocities are increased on account of an 
increase in volume. The gradient of a river, or its average 

^ A demonstration of this proposition may be found in Joseph Le Conte's 
Elements of Geology, 4th edition, pp. 19, 20. Appleton & Co., 1896. 



20 RIVERS OF NORTH AMERICA 

fall in a given distance, as a rule, progressively decreases 
from near its source to its mouth. With this general de- 
crease in gradient there is a decrease in velocity, and con- 
sequently a loss in transporting power and a diminution or 
total check of friction on the stream's bottom. As a result 
of these conditions, we usually find that the streams are 
actively engaged in deepening their channels in their upper 
courses, and are consequently able to extend their branches 
farther and farther, thus acquiring new territory, and at the 
same time to deposit material in their lower and less steep 
courses nearer their mouths. Variations in this process oc- 
cur not only from season to season but from day to day, on 
account principally of variation in velocity due to changes 
in volume. 

The carrying of debris consumes some of the energy of 
flowing water. As an extreme example, it is readily seen 
that an excessive quantity of fine mud contributed to ? 
stream will entirely check its flow. If but little mud is 
added, however, it is carried forward without sensibly 
diminishing the strength of the current. Without attempt- 
ing to present a complete analysis of the laws governing 
stream transportation, it will be sufficient at this time to 
note that streams exert a selective power, taking up and 
carrying forward the finer and lighter material within their 
reach, and, if this be sufficient to consume their available 
energy, leaving the larger and heavier masses, although 
they may not be too heavy to be removed if the energy of 
the stream is not otherwise taxed. 

The principal laws governing stream transportation may 
be briefly formulated as follows: 



LAWS GOVERNING THE STREAMS 2 1 

1. The greater the slope of a stream channel, the greater 
the amount of material in suspension the stream can carry; 
the reason being that the greater the slope the swifter is the 
flow of the water descending it, other conditions remaining 
unchanged. The increase in transporting power with in- 
crease of slope is greater than a single ratio. That is, if the 
declivity of a stream is double, its transporting power is 
more than double. 

2. An increase in the volume of a stream increases its 
ability to transport. The greater the volume of a stream, 
the greater will be its velocity, and the less its loss of power 
due to friction in proportion to its energy. Here, again, the 
increase in transporting power is greater than a simple ratio. 

3. The capacity of a stream to transport is greater for 
fine debris than for coarse; for the reason that to move fine 
material requires less power for the same weight than for 
coarser material, and, also, when the material is fine a 
greater portion of the stream's energy can be utilised than 
when the load is coarse. 

One of the most important principles connected with 
stream transportation is that flowing water assorts the 
debris delivered to it. Fine particles are more easily carried 
than coarser ones of the same specific gravity, and are first 
removed. This is true both of particles in suspension 
and of material rolled along the bottom. If the fine ma- 
terial is sufficient to consume the available energy of a 
stream, all coarser debris is left until its energy is increased, 
as during storms, or until the fragments too large to be re- 
moved are reduced in size. There is a delicate adjustment 
between the velocity of a stream and the size of the debris 



22 RIVERS OF NORTH AMERICA 

it can carry, which may be termed the selective power of 
currents. The influence of this selective power is seen not 
only in the character of the material moved by streams, and 
in the debris left on their bottoms, but in the deposits which 
they make, whether on their border, or in the lakes and sea 
to which they contribute their loads. As most sedimentary 
rocks are formed of stream-born debris, this assorting pro- 
cess must evidently be of vast geological importance. 

Debris Carried by Ice, — An interesting and at times an 
important factor in stream transportation is the assistance 
furnished by ice, which frequently enables streams to move 
objects that would otherwise exceed their power. 

During winter the water of streams frequently freezes to 
the bottom, more especially along their margins, and stones, 
gravel, sand, etc., forming the beds of their channels, be- 
come firmly attached to the ice. In spring, when the 
streams are swollen, the ice, on account of its buoyancy, 
breaks away from the bottom, but frequently retains large 
quantities of debris, which is carried with it down-stream, 
and may make a long journey before being dropped or de- 
posited by the stranding of the ice.^ Stones carried in this 
manner are frequently of large size. A rudely spherical 
boulder measured by me on the bank of the Yukon, which 
had certainly travelled scores of miles from its parent ledge, 
was a little over six feet in diameter. Many others very 
nearly as large were seen which had recently been forced 
several yards up the banks of the river. All of these were 

^ An account of the method of transportation here discussed may be found in 
Lyell's Principles of Geology (nth edition, vol. i., pp. 359-363, Appleton & Co., 
1873), accompanied by an illustration of large boulders along the shores of the 
St. Lawrence, which had been moved through the agency of ice. 



LAWS GOVERNING THE STREAMS 23 

far beyond the reach of former glaciers, and, without ques- 
tion, had been deposited in their present positions at a very 
recent date; some of them, in fact, during the floods of the 
preceding spring. 

Another method by which the ice of a large river some- 
times becomes freighted with debris may be observed where 
high-grade tributaries occur. In such an instance, when 
spring approaches, the small streams may first become freed 
of ice and be able to sweep down debris upon the still frozen 
surface of the river. Again, when a river is bordered by 
steep bluffs, material loosened from the faces of the cliffs 
falls upon the ice and is ready for removal when freshets 
occur. Avalanches may also bring debris to a frozen river 
in the same way that they do to glaciers. 

The assistance in transportation rendered to streams by ice 
is, of course, greatest in high latitudes, but is not inconsider- 
able as far south as Virginia. Along the Potomac there are 
frequently boulders much too large for the unaided waters 
to move, and which it is presumed have been buoyed up by 
ice during a part at least of their journeys, since they are 
well beyond where the river loses velocity on passing from 
its high-grade upper course to the plain near the sea. In 
the method of transportation here considered it is not neces- 
sary that a river should freeze from side to side. The ice 
that forms about a partially submerged boulder near the 
shore of a stream tends to buoy it up. When the water 
surface is raised, if sufificient ice has formed about the stone, 
it will be floated away.* In the terraces of the Potomac 

^ The weight of a cubic foot of water at 32° F. is 62.417 pounds ; a cubic 
foot of ice weighs 57.2 pounds. 



24 RIVERS OF NORTH AMERICA 

about Washington, there are boulders two to three feet or 
more in diameter, resting on fine sand and clay, which it is 
thought were attached to ice-cakes at the time of their re- 
moval to their present sites. The locality referred to, it 
will be remembered, is south of the southern limit of former 
glaciers. 

The stone-charged ice carried each spring by the rivers in 
high latitudes acts much like a glacier in grinding the bot- 
tom and sides of the channel down which it moves. In the 
case of a large river this action is most pronounced near its 
borders, where the water is shallow, and probably does not 
occur at all in the deeper portions. On the border of Por- 
cupine River, Alaska, I have seen large areas exposed 
during low water where the bottom consisted of stones em- 
bedded in tenacious clay so as to form a veritable pavement. 
The ice charged with debris had previously moved over this 
pavement, and not only pressed down the stones so as to 
produce a generally even surface, but ground their exposed 
portions so as to make facets which were polished and 
striated. The pebbles and in some cases flat stones a foot 
or more in diameter, bearing these markings, have a remark- 
able resemblance to glaciated boulders. 

In other instances along the Yukon I found the solid 
rock, on prominent points, to a height of twenty feet or 
more above the summer level of the river, smoothed and 
striated by the action of river ice in much the same manner 
that is familiar in formerly glaciated valleys.* 

During spring floods in northern rivers, ice-blocks are fre- 

^ I. C. Russell, *' Notes on the Surface Geology of Alaska," in Btilletin of 
the Geological Society of America, vol. i., pp. 116-122, 1890. 



Plate 




Views on the Yukon, Alaska. 

A.— Looking from the river across a portion of its delta. 
B. — River-bank of perennially frozen gravel. 
C. and D. — Stones left by floating ice during spring floods. 
E, — Bluff of hard rock on the border of a deeply cut valley. 
F. — Small cut-terraces in sand deposited during high-water. 



LAIVS GOVERNING THE STREAMS 2$ 

quently stranded and even forced far up the banks. Where 
flat cakes of ice accumulate in this manner, they sometimes 
have gravel and sand washed over them ; this material lodges 
in the cracks and openings between the cakes, and is left in 
curious heaps and ridges when the. ice is melted. Sometimes 
these deposits surround small areas so as to make shallow 
basins. 

There is yet another method by which ice assists streams 
in moving debris down their channels, and one which oper- 
ates in midstream, where the current is swift. I refer to 
the formation, during excessively cold weather, of what is 
termed anchor ice, or grouitd icCy at the bottom of streams, 
especially where the waters plunge over small obstructions 
and comparatively quiet bottom-eddies are produced. This 
bottom ice forms about stones, and by its buoyancy tends 
to lift them from the bottom, and thus to assist the currents 
in carrying them away. 

An instructive account of the formation of anchor ice in 
one of the rivers of New Brunswick, during the winter of 
1869-70, has been recorded by W. G. Thompson,' one of 
the engineers of the Intercolonial Railway. In this ac- 
count, quoted below, it is stated that not only were small 
stones lifted from the bottom and floated down-stream, 
but, what is of still greater interest, the ice increased in 
thickness in some instances until it formed dams, and the 
stream was turned from its course. Mr. Thompson's ac- 
count is as follows: 

" The Matapediac, which is fed by large fresh-water springs, 
runs over a rocky bottom covered with loose stones, ranging in 

'^Nature, vol. i., p. 555, 1870. 



26 RIVERS OF NORTH AMERICA 

size from coarse gravel to boulders as large as a hogshead, and 
the average current is about four miles an hour. 

" Early in November last the temperature went down in one 
night to 12° F., and on going out of camp the following morning 
I noticed large quantities of what appeared to be snow saturated 
with water floating down the stream, but not a particle of snow 
had fallen near us for many miles round, as far as I could see 
by the mountain-tops, nor had any ice formed on the surface of 
the river. 

** The water opposite where I stood was about six feet deep, 
and perfectly clear, so that I could see every stone on the bottom, 
and, with the exception of the floating slush, the river was as it had 
been the previous day when the temperature was about 50° F. I 
got into a canoe and paddled with the current for half a mile or 
so, and in shooting some small rapids, where the water in places 
was not more than two or three feet deep, I noticed on the 
bottom masses of the slush clustered round and between the 
boulders, and a slight touch with the paddle was sufficient to free 
these clusters, when they rose to the surface, and were carried 
away by the current. I continued down the stream for three or 
four miles, and noticed the same thing in every rapid, where the 
water was shallow and ruflied by stones at the bottom. 

" The buoyancy of this slush was such that when detached 
from the bottom it rose so rapidly as to force itself well out of 
the water, and then floated off about half submerged. 

" I watched this forming of slush for many days, and in several 
cases found small stones embedded in the floating slush, having 
been torn from the bottom when the buoyancy of the slush, aided 
by the running water, caused it to rise. 

'* The temperature continued getting lower daily, and the slush 
in the rapids formed more rapidly than it was carried away, so 
much so that a bar or dam was formed across the river at each 
rapid, backing up the water in some cases five or six feet, when 
it generally found an outlet over the adjoining land, and into its 
natural bed again, or the head of water became sufficient to tear 
away the obstruction, which by this time had become a solid 
frozen mass. 



LAWS GOVERNING THE STREAMS 2J 

** All this time, no properly crystallised ice had formed on the 
surface of the river, the current being too rapid, but the slush of 
' anchor ice,' as the trappers call it, was forming in deeper 
water than it had formed in before, indeed all over the river 
bottom, and was rising and floating away as I have already de- 
scribed. Eventually the temperature got down to two and three 
degrees below zero, when the river surface began to freeze in the 
eddies and along the edges, and the open-water space became 
narrower every day, and was filled with floating ' anchor ice ' 
and detached masses of solid ice, which here and there became 
jammed and frozen together, so as to form ice-bridges on which 
we could cross. 

" These ice-bridges served as booms to stop much of the float- 
ing ice, which froze solid the moment it came to rest; and in this 
manner the river at last became completely frozen over for about 
forty miles of its length, but not until after we had experienced 
five weeks of steady cold, with the thermometer never above 12" 
F., and frequently down to — 16° F." 

When we recall the fact that the conditions of tem- 
perature described above recur every winter throughout 
nearly one-half of North America, it becomes evident that 
anchor ice must play an important part in stream trans- 
portation. But little attention has been directed to this 
matter, however, and it is highly desirable that someone 
favourably located for such studies should make a careful 
record, especially as to the number and size of the stones 
picked up from the bottom of stream channels, owing to 
the buoyancy of the ice formed about them. It will be 
noted that this process of transportation is brought into 
operation simply by a lowering of temperature, and does 
not require a rise of the water in order to float the debris 
attached to the ice, as is the case when surface ice becomes 
fastened to the bottom. Anchor ice operates in the way 



28 RIVERS OF NORTH AMERICA 

described in midstream, where the water is not only swift 
but may be comparatively deep ; while the similar work of 
surface ice is practically confined to the shallow water on 
the margins of streams. 

Corrasion, — Clear streams, as we ordinarily see them, are 
such as have removed from their channels all of the particles 
within their reach which they are competent to transport, 
although they may still roll and push coarse fragments along 
their bottoms. It is to be noted, however, that clear 
streams, when of a given velocity, may become muddy if the 
velocity is increased. The friction of clear running water 
is but slight, hence streams not charged with material in 
suspension wear their channels very slowly. In such in- 
stances chemical solution of the rocks over which the waters 
flow may be in excess of mechanical abrasion. 

When a stream receives an initial load of silt and sand 
from rain-wash, the action of the wind, glacial abrasion, 
etc., an important change in its behaviour occurs. The 
transported fragments on being brought in contact with the 
bottom and sides of the channel of the stream produce 
abrasion. The flowing waters charged with silt and sand 
act not unlike a strip of sandpaper that is drawn over an 
object continually in one direction. The transported frag- 
ments abrade the rocks with which they are brought in 
contact, and are themselves worn and broken. Gravel and 
larger rock fragments too heavy to be carried in suspension, 
except during floods, are worn and broken by smaller frag- 
ments coming in contact with them, and when moved during 
high-water stages, assist in a marked way in promoting the 
process of channel enlargement. The friction and impact 



LAWS GOVERNING THE STREAMS 29 

of the particles that are carried forward tend to loosen and 
dislodge other fragments, and thus increase the amount of 
material available for transportation. 

It is convenient to consider the process of stream abrasion 
due to the friction of transported material and to chemical 
solution, as a part of the general process of land degradation, 
and to give it a separate name. To meet this want, the 
term corrasion has been proposed.^ 

The deepening and widening of a stream channel — that is, 
corrasion — is carried on mainly by mechanical wear due to 
the friction of silt, sand, gravel, boulders, etc., carried 
through it by the flowing waters, but is assisted, many 
times in an important manner, by solution. 

There are conditions that limit or modify the competency 
of a stream to transport debris, as has already been briefly 
considered. In a similar way there are conditions which 
modify and limit the rate of stream corrasion. Streams, as 
is well known, vary in rate of flow, in volume, in declivity, 
in the degree to which they are loaded, in chemical com- 
position, etc. Changes in any one of these conditions will 
manifestly exert an influence on the rate at which a stream 
is enabled to deepen and widen its channel. 

The nature of the load carried by a stream also varies in 
different instances, and even from month to month and 

^ As the nomenclature of dynamical geology and physical geography is not 
yet definitely fixed, it may be suggested that corrasion furnishes a convenient 
generic term, and may be made to include the processes of abrasion by stream- 
like movements of other substances than water, when charged with rock 
fragments. The grinding of rocks by glaciers may be designated as glacial 
corrasion ; the process of wearing of rocks by dust and sand transported by air- 
currents becomes aolian corrasion ; and lake and ocean shores furnish examples 
of wave and current corrasion. 



30 J^IVERS OF NORTH AMERICA 

from day to day in the same stream. The particles or frag- 
ments carried are fine or coarse, hard or soft, rounded or 
angular; all of these conditions have an influence on the 
amount of friction exerted on the stream bed, and hence 
modify the rate of corrasion. Again, the rate at which a 
stream channel is enlarged under the supposition that the 
velocity of the stream, the character of its load, etc., remain 
constant, will vary with the nature of the rocks over which 
it flows. Hard rocks are worn more slowly than soft rocks, 
easily soluble rocks are more rapidly removed than those of 
difficult solubility. There are still other conditions pertain- 
ing to the beds of streams which influence corrasion. Mas- 
sive rocks yield less readily than those perhaps of equal 
hardness and equally soluble, but which are much jointed, or 
occur in thin layers. Rock texture thus exerts an import- 
ant influence on corrasion, as does also the inclination of 
the rocks or their dip. When hard and soft beds alternate, 
other conditions being the same, corrasion is more rapid 
when they are inclined than when they are horizontal. 

It is unnecessary to trace the effects of variations in these 
several conditions, as the student may do this for himself, 
and thus have the pleasure of making independent dis- 
coveries. In the case of a stream flowing under stated con- 
ditions, let the student postulate an increase in volume, in 
declivity, in character of load, in hardness of the rocks 
forming its channel, etc., other conditions remaining the 
same in each case, except so far as the reaction on them of 
the postulated change is concerned, and trace the effects on 
the rate of corrasion. 

The principal laws governing corrasion are, briefly: 



LAWS GOVERNING THE STREAMS 3 I 

1. The rate at which a stream corrades its channel, other 
conditions remaining the same, increases with increase in 
load to a certain point which varies with the character of 
the load. If the load continues to increase, friction on the 
bottom decreases, and ceases when the entire energy of the 
stream is consumed in transportation. 

2. Other conditions remaining the same, corrasion in- 
creases with declivity and with volume of water, since each 
of the changes increases velocity. 

For a more detailed discussion of the laws governing 
stream transportation and corrasion than it is practicable to 
present at this time, the reader is referred to Gilbert's * ad- 
mirable analysis of land sculpture, already cited many times 
in the present treatise. 

The conditions controlling the amount of detritus a stream 
can transport are mainly velocity and volume. Velocity is 
increased by an increase in volume and also by increased 
declivity. In nature we find that streams ordinarily vary 
in volume with seasonal changes and also from day to 
day, and hence their ability to transport, and consequently 
to corrade, undergoes many fluctuations. . The gradients 
of stream channels vary from place to place along their 
courses, and hence their ability to deepen their channels is 
not the same in all parts. The load that a stream carries in 
one portion of its course may be too great a burden in 
another portion, and some of it, always the coarser portion, 
will be dropped. The journeys of stream-borne debris are 
thus far from being continuous. The transported material 

' G. K. Gilbert, Report on the Geology of the Henry Mountains, 4to, pp. gg- 
150. Department of the Interior, U. S. Geographical and Geological Survey 
of the Rocky Mountain Region, J. W. Powell in charge. Washington, 1877. 



32 RIVERS OF NORTH AMERICA 

is laid aside from time to time in bars and flood-plains. It 
may require tens of thousands of years for a given rock 
fragment loosened on a mountain-side to reach its final 
resting-place in the sea. 

In nature we find, as a rule, that the gradients of streams 
decrease from their sources to their mouths, but it must be 
remembered that this is the result of the action of the 
streams themselves and follows a long period of development 
and adjustment. As increased declivity favours corrasion, 
it is to be expected that the mountain tracts of streams 
will be deepened at a greater rate than their valley tracts, 
and that they will be enabled to extend their branches 
farther and farther, and thus acquire new territory. This, 
in fact, is the case, as we know, for the great majority of 
rivers are corrading their channels in the highlands and de- 
positing in the lowlands. Rivers are ordinarily supplied by 
many branches, however, which means that the volume of 
water in the branches is less than in the trunk stream, and 
accompanying decreased volume, other conditions remain- 
ing the same, is a decreased corrasion. The behaviour of 
a stream in reference to corrasion and deposition is thus a 
resultant of many and frequently opposing conditions. As 
will be shown later, the ability of a stream to corrade or de- 
posit in a given portion of its course varies ordinarily with 
its age, or, more accurately, with its stage of development. 
The portion of a stream channel where corrasion is in active 
progress during its youth, may become a region of deposi- 
tion at a more advanced stage in its history under the pro- 
cess of normal development which streams experience even 
if no changes occur in land elevation. 



LAWS GOVERNING THE STREAMS 



33 



Pot-Holcs. — One of the minor phases of stream corrasion is 
illustrated by the cylindrical holes frequently worn in the 
beds of streams by stones swept about by strong currents. 
These holes are sometimes saucer-shaped, but more fre- 
quently have steep sides and rounded bottoms, and resemble 
the insides of the familiar cast-iron kettles used for culinary 
purposes ; this similarity has suggested the name pot-hole, by 
which they are commonly designated. Their walls are 
usually smooth, and sometimes exhibit grooves and ridges 




Fig. I. A Pot-Hole being Scoured out by Stream Action. 
(After R. S. Tarr.) 

in horizontal planes or arranged more or less spirally. In 
these grooved holes one sometimes finds well-worn pebbles, - 
or even large boulders, and discovers a relation between the 
size of the grooves and of the stones that made them. In 
many instances, also, these mills are still in working order, 
and a stream of water is plunging into them, as is shown in 
the accompanying photograph. 

3 



34 RIVERS OF NORTH AMERICA 

Pot-holes are of all sizes, from shallow depressions a few- 
inches in depth to vertical borings five or six feet across and 
fifteen or twenty feet or more deep. From these well de- 
fined examples there is a gradation up to the basins pro- 
duced by the stones swirled about at the bases of waterfalls, 
as the pool into which Niagara plunges, for example, which 
might perhaps be termed compound pot-holes. 

In the making of these characteristic depressions the mill- 
stones may be worn out, but new ones are supplied from 
time to time, and the process goes on. Similar excavations 
are made also beneath glaciers, where streams flowing on 
the surface of the ice plunge into crevasses, or deep well- 
like openings termed moulinSy and reach the rocks below. 
In fact, any strong current by being deflected may cause 
loose stones to be whirled about so as to grind the rocks on 
which they rest, and produce depressions of the nature here 
considered. Favourable conditions result when pebbles 
and boulders of hard material occur in a stream where the 
bottom is of soft rock, and where also the current is swift, 
and eddies, swirls, whirlpools, etc., are produced. 

Lateral Corrasion, — Streams in moving material along 
their channels not only wear away the rocks over which 
they flow, but abrade the sides of their channels as well. 

It is difficult to formulate the laws governing lateral cor- 
rasion, but the general manner in which it is accomplished 
may be readily understood. 

In considering the process of vertical corrasion the influ- 
ence of upward currents in flowing water was recognised. 
But besides the upward currents there are also lateral cur- 
rents. In fact, the flow of water, even through smooth, 



LAWS GOVERNING THE STREAMS 35 

ft 
straight troughs, is complex, and many secondary currents 

are generated. This complexity is vastly increased when 
the channel is rough and irregular. If we watch a swift- 
flowing stream, we shall be enabled to see that there are 
many swirls and eddies due to, or accompanied by, currents 
moving in all directions. Along the sides of a stream 
channel the secondary currents strike the rocks and dash 
against them whatever material the waters may hold in sus- 
pension. Friction results from the impact of the floating 
particles, and tends to wear away the sides of the channel. 
The larger fragments rolled along the bottom of the stream 
can take but little part, directly, in this process of channel- 
widening. It is the finer material — the silt and sand held 
in suspension — which does most of the work. Moreover, 
the courses of stream channels are seldom, if ever, straight 
for any considerable distance, but are a succession of con- 
cave and convex curves. The water is alternately thrown 
against one bank, and the direction of the current being de- 
flected, impinges on the opposite bank lower down-stream. 
At the locality where the thread of swiftest flow nears the 
bank, the rocks are worn away, and the irregularity of the 
stream's course increased. The material removed is carried 
down-stream, and in part deposited in the slack water on 
the concave side of the next bend. Swift streams are not 
so easily turned aside as those which flow less impetuously. 
Hence the former maintain straighter courses than the 
latter. 

Lateral corrasion may go on when vertical corrasion is 
checked by decrease in declivity, deposition, or for other 
reasons. After a stream has cut down its channel at its 



36 RIVERS OF NORTH AMERICA 

mouth to the level of the still water into which it discharges 
and established a low gradient, lateral corrasion may con- 
tinue. It is under these conditions that the widening of 
river valleys is mainly accomplished. In general, vertical is 
so far in excess of lateral corrasion in the case of high-grade 
streams, that the valleys produced are narrow, and V-shaped 
in cross-section, while under similar conditions in respect to 
climate, rock texture, etc., in a low-grade stream, although 
its actual rate of lateral corrasion may be less than in the first 
instance cited, the ratio of lateral to vertical corrasion is 
greater, and a flat-bottomed valley results. The cross-pro- 
file of a valley widened by lateral corrasion is U-shaped, and 
if the process is long continued becomes broad-bottomed. 
The statement frequently made that a stream-cut valley is 
V-shaped in cross-profile, in distinction from the U-shape 
of valleys formerly occupied by glaciers, is not strictly true, 
as it considers only young stream-valleys. 

Meanderifig Streams, — The serpentine courses followed 
especially by sluggish streams, just referred to, is a matter 
of more than passing interest to the student of geography. 
Much of the charmingly picturesque which enters into and 
many times forms the leading feature of stream-side scenery, 
as well as the secret of the process by w^hich valleys are 
broadened, and the adjacent uplands removed, results from 
the meandering and lateral migration of streams. By this 
same general process, too, as will be considered later, the 
flood-plains of rivers are spread out. Illustrations of the 
curves characteristic of many streams, are given on Plate 3. 

The causes leading to the meandering of streams have 
been studied by various observers. As stated by Fergus- 



LAPVS GOVERNING THE STREAMS 37 

son/ a river is a body of water in unstable equilibrium, 
whose normal condition is that of motion down an inclined 
plane, and if all inequalities in the material forming the 
bottom and sides of its channel could be removed, it would 
flow continuously in a straight line. (It is to be noted, 
however, that the influence of the earth^s rotation is not 
considered in this discussion.) Any obstruction or inequal- 
ity, however, necessarily induces an oscillation, and, the 
action being continuous, the effects are cumulative, and 
the oscillation goes on increasing till it reaches a mean be- 
tween the force of gravity tending to direct the current in a 
straight line, and the force due to the obstruction tending 
to give a direction more or less at right angles to the former. 
In nature not one but many disturbing conditions occur, 
and the streams flow in a series of curves, each of which 
bears a definite relation to their volumes and the gradients 
of their channels. 

The stage in the lives of rivers when they meander in 
broad curves through rich bottom-lands, is usually, or most 
commonly, reached late in their lives, when the task of re- 
ducing their channels to the level of the still water into 
which they discharge has been nearly completed. They 
then flow sluggishly, and may be said to be enjoying the 
rest to which a long life of activity entitles them. A slack 
current and a tortuous course are not infallible indications 
of old age, however. Young streams flowing across an 
abandoned lake bed, for example, or over lands recently 
raised from the sea, may have these characteristics. A 

'James Fergusson, *' On Recent Changes in the Delta of the Ganges," in 
The Quarterly Journal of the Geological Society of Londo7i, vol. xix., p. 323, 

1S63. 



38 RIVERS OF NORTH AMERICA 

winding course may be retained by a river which has been 
given renewed energy by a re-elevation of the land, even 
after it has cut a deep trench and is a hurrying torrent. 
The tendency to meander is strongest in streams that are 
heavily loaded and are depositing a portion of their burdens 
in flood-plains. 

Although the tendency to meander characterises all 
streams, for the reason that their channels are never 
straight or composed of homogeneous material for any con- 
siderable distance, yet the process carries with it certain 
limitations, as will be shown in discussing the origin and 
nature of flood-plains and terraces in a subsequent chapter. 

Other Curves, — Not only do streams bend to the right 
and left of their general courses, as where a river meanders 
through a broad, partially alluvial-filled valley, but, as will 
be described later, form curves in a vertical plane as well. 
Where corrasion is in progress, the longitudinal profile of 
the channel produced is concave to the sky, and where de- 
position occurs, curves convex upward result. More or less 
complete spiral curves about vertical or inclined axes, like 
the twists of a corkscrew, may be seen when a high-grade 
stream is excavating soft clays, and where a brook on the 
surface of a glacier plunges into a well-like opening in the 
ice. The influence of the graceful sweep of stream-curves, 
on the beauty of many landscapes, is due to their infinite 
variety; no two in the course of even a great river being 
identical. This marvellous diversity, produced by simple 
means, becomes still more impressive when it is remembered 
that no one of these many curves remains the same for any 
considerable period of time. 



Plate III. 



^N^.^'.\. uJJCkL ' . ..'i^^'H^.k 




Fig. a. Ray Brook, Adirondacks, New York. 

(Photograph by S. R. Stoddard.) 




Fig. B. Moccasin Bend, Tennessee River, from Lookout Mountain ; 
Chattanooga at the Right. 



LAM^S GOVERNING THE STREAMS 39 

The several classes of curves in the channels of streams 
are supplemented in an interesting manner by other curves 
in the surfaces of the streams themselves. Not only are 
graceful curves produced by the flow of water in eddies and 
swirls, or when they arch over or circle about obstacles, 
and are thrown into waves by the wind and other causes, 
but the surface of a stream in cross-section is not a straight 
line, although this condition is very nearly reached when 
the current is gentle, and the waters deep. If there is a 
strong central current, however, the surface there forms 
a convex curve, which rises well above the more gently 
flowing waters on either side. In swift rivers this difference 
in level frequently amounts to five or six feet, and in certain 
instances, as at the whirlpool below Niagara, is reported to 
be two or three times these measures. In such examples 
the surface line of a cross-profile would show a pronounced 
upward curve in the central part, bordered on each side by 
a downward curve. The bordering downward curves are 
gentle, but may be recognised, although they probably de- 
part but little from a straight line. 

Driftwood carried by a stream with a swift central current, 
as will be described more fully in advance, tends to leave 
the elevated central part and collect along the banks. This 
tendency may be seen especially when swift streams are 
rising; when the waters fall, however, driftwood leaves the 
slack water adjacent to the shore, and tends to concentrate 
in mid-stream. At such times the stream in cross-profile 
probably presents a concave surface-line. 

Deflection of Streams Owing to the Rotation of the Earth, — 
The earth, as is well known, makes one rotation from west 



40 RIVERS OF NORTH AMERICA 

to east about an axis passing through the poles, in 23 hours 
56 minutes and 4 seconds. The circumference of the earth 
being about 24,000 miles, any point on the equator must, 
therefore, travel over 1000 miles an hour. North and south 
of the equator this motion gradually decreases, and becomes 
zero at the poles. This motion has an influence on the flow 
of streams, and tends to cause them to follow curved instead 
of straight courses. This may be most readily understood 
in the case of streams having either north or south direc- 
tions, but affects all streams on the earth's surface, unless 
they follow strictly the path marked out by the equator. 

Water flowing northward from the equator would start 
with the motion from west to east, which pertains to that 
location, but as it advanced would cross regions which 
have progressively less and less motion from west ta 
east. The current due to gravity, we will assume, tends 
due north, but the waters have also a motion from west to 
east, due to the earth's rotation, which is in excess of the 
similar motion of the region necessarily invaded. The re- 
sultant of these two forces will carry the stream to the east 
of the meridian on which it started, and the stream will curve 
to the right of its initial course. In a similar way, a stream 
in the northern hemisphere, flowing toward the equator, 
would be continually invading territory having greater and 
greater motion from west to east and would curve to the 
west of the path it would follow if influenced only by gravity. 
Thus, in the northern hemisphere, the tendency of the 
earth's rotation is to cause the streams, no matter what 
their direction of flow, to corrade their right more than 
their left banks. In the southern hemisphere the direction 



LAWS GOVERNING THE STREAMS 4 1 

of curvature due to the earth's rotation is reversed, and the 
streams, no matter what their direction, tend to corrade 
their left more than their right banks. This is an applica- 
tion to streams of Ferrel's law, namely: '' If a body moves 
in any direction on the eartlis surface, tJiere is a deflecting 
force arising front the earth' s rotation whicli deflects it to the 
right in the nortliern hemisphere, but to the left in the southern 
hemisphere,'' * 

The tendency of a stream to maintain a straight course as 
it invades territory having a progressively changing rate of 
motion is greater the greater the velocity of the stream ; the 
same is true of the parts of a stream. The thread of maxi- 
mum current in a stream following an approximately straight 
course, is in the centre near the surface. The change in 
direction owing to rotation is, therefore, less quickly mani- 
fest in the thread of maximum current than in the more 
sluggish waters on either side, and the current undergoes 
greater deflection. Where streams follow winding courses 
this tendency leads to an increase in their meanderings to 
the right, in the northern hemisphere, more than to the left 
of their general direction. There is thus a tendency, due to 
the earth's rotation, for them to excavate their right more 
than their left banks, and to migrate to the right of their 
initial courses. This tendency is slight, but all the time 
operative. Owing, however, to inequalities in hardness of 
the banks of streams, and other disturbing conditions, it is 
difficult to discover examples where the earth's rotation has 

^ William Ferrel, " The Motions of Fluids and Solids Relative to the Earth's 
Surface," in The Matheviatical Monthly, vol. i., p. 307, 1859. ^ he influence 
of the earth's rotation on air-currents is clearly explained in W. M. Davis's 
Elementary Meteorology^ pp. loi-iii. Ginn & Co., Boston, 1894. 



42 RIVERS OF NORTH AMERICA 

plainly controlled their migrations. An illustration of such 
a result is thought to be furnished, however, by the streams 
on the south side of Long Island, where there is a plain 
with a remarkably even descent and gentle slope. This 
plain is crossed by a number of small streams which have 
excavated shallow valleys in essentially homogeneous 
gravel. Each of these little valleys is bordered on the 
west, or right side, by a bluff from ten to twenty feet high, 
while its gentle slope on the left side merges imperceptibly 
with the general plain. The stream in each case follows 
closely the bluff at the right. As stated by Elias Lewis, 
and affirmed by Gilbert,^ there seems to be no room for 
reasonable doubt that these peculiar features result from 
the influence of terrestrial rotation. 

It is to be remembered that the force of rotation, like 
gravity, is all the time operative, but its influence is greatest 
on streams flowing north or south, is greater in high than 
in low latitudes, and increases with the rapidity with which 
the waters are transferred from an area having a certain 
motion to another area having a different motion. The 
results of this influence, although not conspicuous, are nev- 
ertheless important. There is a slight tendency through- 
out the length of every stream in America and at all times, 
to erode its right more rapidly than its left bank. In the 
case of the Mississippi, shown by Gilbert in the article 

' G. K. Gilbert, " The Sufficiency of Terrestrial Rotation for the Deflection 
of Streams," in American Journal of Science, vol. xxvii., pp. 427-432, 3d 
series, 1884, An abstract of this paper, accompanied by an extension of the 
discussion, may be found in Science, vol. iv., pp. 28, 29, 1884. See, also, \V. 
M. Davis, " An Early Statement of the Deflective Effect of the Earth's Rota- 
tion," in Science, vol. i., p. 98, 1S83. 



ZAPVS GOVERNING THE STREAMS 43 

cited above, the selective tendency thus determined toward 
the right bank is nearly nine per cent, greater than toward 
the left bank. 

Questioning the Rivers, — To illustrate the laws govern- 
ing the behaviour of streams, let us see how some of the 
leading features of the rivers of North America can be 
accounted for. 

Why, for example, are the waters of the St. Lawrence 
clear and those of the Missouri usually muddy ? The 
former is obviously a clear stream for the reason that the 
Great Lakes it drains act as settling basins and retain the 
sediment brought down by countless tributaries. Many 
small streams join the St. Lawrence below Lake Ontario, 
but most of these also have lakes in their courses, and the 
amount of the sediment reaching the main river from rills 
and brooks is not sufficient to materially change its charac- 
ter. The' clear waters of the St. Lawrence have but little 
power to corrade. The current is swift in places, but all of 
the fragments in its bed which the current is competent 
to move have long since been carried away. Corrasion has 
gone on with extreme slowness throughout the present 
geographical cycle, and the river has not yet entrenched 
itself, but is practically a surface stream. 

The reader will, no doubt, at once remark that the Mis- 
souri and the Platte are also surface streams, although heav- 
ily loaded with silt and sand. These rivers, however, rise 
in high mountains and flow across a broad plateau. Their 
many branches in the mountains are swift and bear along 
heavy loads of detritus. On leaving the mountains and 
entering their plain tracts, velocity is checked, and the less 



44 RIVERS OF NORTH AMERICA 

swift waters are no longer able to carry the loads they pre- 
viously transported with ease, and deposition occurs. Then, 
too, the surfaces of the Great Plateaus are composed of 
easily eroded rocks. During every rain quantities of soil, 
etc., are washed into the rivers, and during the long, dry 
summers, the winds are busy in performing a similar task. 
At present not only is corrasion nil throughout the plain 
tracts of the Missouri and the Platte, but sedimentation is 
in progress. These rivers are aggrading previously eroded 
valleys. 

The question of how deep and how wide a river valley 
shall be, depends not alone on the elevation of the land 
above sea-level, but also on the ratio of stream corrasion 
to the general waste or erosion of the bordering lands. 
When the rate of stream corrasion is in marked excess of 
the rate at which the general surface of the land is being 
eroded, deep, narrow stream channels result. But if the 
erosion progresses at nearly the same rate as corrasion, the 
relief of the region will be mild. Between these two ex- 
tremes there are many intermediate stages. Overloaded 
streams, by dropping a portion of their burdens, may 
not only spread out broad flood-plains, but also elevate 
their channels so as to flow at a higher level than the sur- 
rounding land. The Platte and the Missouri are now de- 
positing material throughout their courses across the Great 
Plateaus, and, as just stated, are aggrading previously 
formed broad-bottomed valleys. 

Many conditions besides those just noticed exert an influ- 
ence on the character and expression of stream-cut valleys. 
When the rocks are hard, they tend to form precipitous 



LAWS GOVERNING THE STREAMS 45 

bluffs; when soft, they crumble, and the valleys have flaring 
sides. When the climate is arid, the wasting away of cliffs 
is long delayed, and if corrasion is actively progressing, 
steep-sided gorges, or canyons, result. Vegetation retards 
surface erosion, although favouring rock decay, and has a 
varied influence on the lives and character of the streams. 
These and still other influences modify the results of stream 
corrasion, which find expression in the scenery of the land. 
It may be asked, why is it that the Colorado has carved 
the most magnificent canyon in North America, while the 
Platte, rising in the same mountains, is bordered throughout 
much of its course by perhaps the least picturesque scenery 
of any large river on the continent ? Much of the answer has 
already been given. The Platte, as we have seen, is aggrad- 
ing its channel to the east of the Rocky Mountains. The 
Colorado is still corrading from its source, with the excep- 
tion of certain slack-water reaches in soft rocks, all the way 
to its place of discharge. The rocks through which the 
Platte flows are soft ; while those cut by the Colorado are 
hard. The region of the High Plateaus crossed by the 
Colorado has experienced a somewhat recent uplift of several 
thousand feet ; while the country traversed by the Platte to 
the east of the Rocky Mountains has, so far as is known, 
undergone but slight changes in elevation during the same 
period. The climate of the Colorado region is arid, and 
surface waste from rains and the action of the wind probably 
less than in the region of the Platte. It is in these and per- 
haps still other contrasts of conditions that the striking 
differences in the scenery along the border of these two 
rivers may be accounted for. 



46 RIVERS OF NORTH AMERICA 

By comparing the scenery of various other regions and 
seeking for the underlying causes to which their differences 
are due, the student may arrive at a juster appreciation of 
the characteristics of river work than a formal statement of 
the subject will furnish. 

Erosion, — Weathering, transportation, and corrasion ^re 
three agencies which by their combined action lead to the 
removal of land upraised above the sea, and the production 
of a vast array of ever-changing topographic forms. To 
this far-reaching and highly complex process, the name 
erosion has been given. 

Briefly stated, the removal of material from land areas is 
accomplished by: ist. The disintegration of the rocks by 
both mechanical and chemical means, through the action of 
the varied and complex process termed weathering, 2d. 
The removal especially of the finer products produced by 
weathering, by the wind, general rain-wash, and rills, and of 
both fine and coarse debris by brooks and rivers, by a pro- 
cess termed trajisportation. 3d. The friction and impact 
of the* material transported, accompanied by solution, lead 
to the deepening and broadening of stream channels, or 
c or r a si 071, 

A necessary accompaniment of erosion is the deposition 
of the material removed, or sedimentation. A temporary 
phase of sedimentation is the laying aside of stream-carried 
detritus in flood-plains and stream channels, but its final 
resting-place, so far as a single geographical cycle is in- 
volved, is on the floor of the sea. 

Baselevel of Erosion. — The depth to which a stream, flow- 
ing into a lake or the sea, can lower its channel by mechani- 



LAWS GOVERNING THE STREAMS 4/ 

cal corrasion is limited by the surface level of the receiving 
water-body. As mechanical corrasion decreases in more 
than a simple ratio with decrease in declivity, the final 
stages in the lowering of a stream channel to the level of 
the still water into which it flows must be extremely slow. 
Corrasion is not limited to mechanical processes, however, 
but includes solution as well. The final reduction of a 
stream channel to sea-level must, therefore, be by solution. 
Every stream, when its entire history is reviewed, will be 
found to be engaged in deepening its channel to the horizon 
referred to, or else has accomplished a part or the whole of 
the task. The datum-plane limiting downward corrasion is 
reached first at the mouth of a stream and is then continued 
progressively towards its source. When many streams are 
considered, various stages in this process may be recognised. 
The depths to which streams may excavate their channels 
is evidently a matter of great importance in their develop- 
ment and in the history of the topographic changes of the 
land. This fundamental principle was first clearly defined 
by Powell,^ who employed the term baselevel to designate 
the lower limit of stream action. It is the baselevel of cor- 
rasion. A lake determines the baselevel for the streams 
flowing into it, but as lakes are short-lived, the real base- 
level toward which all streams are working is the surface 
level of the sea. 

To use Powell's own words in this connection: 

" We may consider the level of the sea to be a grand base- 
level, below which the dry land cannot be eroded; but we may 

^ J. W. Powell, Exploration of the Colorado River of the West and its 
Tributaries, p. 203, 4to. Washington, D. C, 1875. 



48 RIVERS OF NORTH AMERICA 

also have, for local and temporary purposes, other baselevels of 
erosion, which are the levels of the beds of the principal streams 
which carry away the products of erosion. I take some liberty 
in using the term level in this connection, as the action of a run- 
ning stream in wearing its channel ceases, for all practical pur- 
poses, before its bed has quite reached the level of the lower end 
of the stream. What I have called the baselevel would, in fact, 
be an imaginary surface, inclining slightly in all its parts toward 
the lower end of the principal stream draining the area through 
which the level is supposed to extend, or having the inclina- 
tion of its path raised in direction as determined by tributary 
streams." 

When a stream has lowered its channel nearly to base- 
level, downward corrasion is retarded, but lateral corrasion 
continues. Low-grade streams, as we have seen, are the 
ones most inclined to meander, and to broaden their valleys. 
If this process is continued for a sufficient time in any 
region, it will lead to the removal of all land within reach of 
the streams, down to their own level. Baselevel of corrasion 
thus becomes practically the baselevel of erosion. The ulti- 
mate result of erosion is to reduce a land area to a plain at 
sea-level. Such perfect plains, however, are exceedingly 
rare, but approximations to the ultimate result are common, 
and plains in this penultimate stage have been named pejie- 
plains by Davis. 

A peneplain is the normal result of the erosion of the 
land, provided elevation or depression do not occur to 
check the process. If a portion of the earth crust remains 
essentially stable for a sufficient length of time to allow the 
streams flowing from it to broaden their channels, a more 
or less extensive peneplain is produced. Such periods of 

% 



LAWS GOVERNING THE STREAMS 49 

stability, when movements in the earth's crust are not suffi- 
cient to check the normal process of baselevelling, are termed 
geograpliical cycles, A peneplain may be defined as an ap- 
proximately perfect plain produced by erosion during a 
geographical cycle. If, after a portion of a land area has 
been reduced to a peneplain, elevation occurs, a new geo- 
graphical cycle will be initiated, and the process of base- 
levelling again begun. 

During one geographical cycle all of the land may not be 
reduced to a plain, but isolated uplands between broad 
valleys remain. These remnants are left as an inheritance 
to the next succeeding geographical cycle. Mount Monad- 
nock, in southern New Hampshire, is an example of such a 
remnant, and forms a prominent feature on the surface of 
an elevated peneplain. The study of the relief of the land 
in various regions has shown that there are many such rem- 
nants, left by incomplete planation. A name is needed 
for such topographic features, and to meet this, want Davis 
has proposed that they be termed monadnocks, after the 
typical example just referred to. A monadnock, then, is a 
hill or mountain left standing on a peneplain, owing to 
incomplete planation. 

The laws, just stated, governing the reduction of land 
areas to baselevel, although wide-reaching and fundamental 
to the student of geography, are not strictly true, or, rather, 
exceptions to them may be found. The downward limit of 
mechanical corrasion is not in all cases the sea-level. Gla- 
ciers may enter the sea and continue their destructive work 
at a depth of a few hundred feet below its surface. Ice- 
bergs may also disturb the bottom of the sea at considerable 



50 RIVERS OF NORTH AMERICA 

depths. Currents in the sea sometimes corrade the bot- 
tom ; the downward limit to which this process may be 
carried is unknown, but may be hundreds of fathoms. The 
removal of rocks in solution may be carried on deep below 
sea-level; the lower limit has not been determined, but is 
certainly many thousands of feet. Change in the position 
of rock material through the agency of plants and animals 
is not limited downward, by the surface level of the sea, but 
goes on below that horizon. All of these processes, how- 
ever, are of minor importance, and need not be considered 
as sensibly modifying the conclusion that the downward 
limit to which land areas may be reduced is the horizon of 
the surface of the sea. Strictly speaking, baselevel is the 
lower limit of the mechanical corrasion of streams, but prac- 
tically, as we say, it is also the downward limit of erosion. 

Influence of Vegetation on Erosion, — The influence of 
vegetation on the general process of denudation is varied, 
and both retards and accelerates the process. Vegetation 
breaks the force with which rain-drops strike the earth, and 
besides, when the ground is covered with leaves, or with 
grasses, moss, or other plants of low growth, the surface 
w^aters are filtered of such debris as they may have taken 
in suspension. Vegetation thus retards transportation and 
decreases mechanical corrasion. On the other hand, vegeta- 
tion furnishes the percolating water with organic acids, 
principally humus acids, which greatly enhance their solvent 
power. Hence vegetation favours chemical corrasion in a 
high degree. 

The roots of plants bind the soil together, and thus assist 
it in resisting mechanical agencies tending to remove it. 



LAWS GOVERNING THE STREAMS 5 1 

But roots furnish organic acids as they decay, and besides 
open passageways for descending water, thus facilitating 
chemical changes. The student may readily observe other 
modifications in the lives of streams due to vegetation and 
to climate. Interesting results would no doubt be obtained 
from the study of the vegetation which grows in the streams 
themselves, such as the algae and certain higher forms of 
plant life. The direct influence of driftwood and of fallen 
timber is considered in a subsequent chapter. 

Although the summary of the laws governing the be- 
haviour and work of streams just presented is confessedly 
incomplete, yet it is the wTiter's hope that it will serve to 
interest the reader in the processes of land sculpture nearly 
everywhere in progress where the earth's surface rises above 
the sea, and suggest questions to which more technical 
treatises, or, better still, the rills and rivers themselves will 
furnish answers. 

Note. — Since this chapter was written, a highly instructive paper by Hunt- 
ington Hooker has appeared, on " The Suspension of Solids in Flowing 
Water," Transactions of ihe Americait Society of Civil Engineers, vol. xxxvi., 
1897, pp. 239-340, which the reader is recommended to study. 



CHAPTER III 

INFLUENCE OF INEQUALITIES IN THE HARD- 
NESS OF ROCKS ON RIVER-SIDE SCENERY 

IF we watch a hillside rill which is born during a heavy 
shower and runs dry when the sun again shines, it will 
be noted that in the steeper portion of its descent it cuts a 
narrow trench and deposits much of the material removed 
farther down its course. It soon becomes apparent that the 
little stream is corrading where its descent is steep, and 
raising its bed by depositing the material brought from 
above where the grade becomes gentle. The processes of 
corrasion, transportation, and deposition may all be seen in 
operation in a stream only a few rods in length. The topo- 
graphic forms resulting are, on a minute scale, the same as 
those which give grandeur to many far-reaching views of 
river-side scenery. 

The process of excavation and deposition carried on by 
a rill leads to the making of a uniform grade down which 
the waters continue to carry debris. This gradient, as will 
be shown later in discussing the profiles of streams, is not 
an inclined plane, but in longitudinal profile is a curve, 
steepest near the source of the stream, and flattening out 
and approaching nearer and nearer a straight line the nearer 

52 



INEQUALITIES IN THE HARDNESS OF ROCKS 53 

the mouth of the rill is approached. The immediate task 
of the rill may be said to be the making of a certain gradient, 
which is best suited to its volume and other conditions. 

In the portion of the rill channel where corrasion is in 
progress, the waters are broken by little cascades and mini- 
ature rapids, and it is evident that a uniform gradient in 
that portion of its bed is far from perfect. The reason is 
that the clay, earth, or other material in which the rill is 
working is of varying degrees of hardness. When a boulder 
is crossed, a cascade results. When the material is soft the 
channel is quickly deepened. Each passage from a hard to 
a soft layer is marked by a cascade. Evidently the charac- 
ter of the material which the rill is removing exerts a con- 
trolling influence on the miniature scenery along all of its 
upper course. 

If we enlarge our field of observation, similar characteris- 
tics will be found in many brooks, creeks, and rivers. The 
study of many streams will soon show, however, that some 
of them are broken by cascades and rapids, while others, 
except on their extreme headwaters, have even descents 
and flow with a generally uniform current. Those which 
are broken by cascades, we soon learn, for the most part oc- 
cupy narrow, steep-sided trenches, and are apt to have lakes 
along their courses ; while those with a uniform descent from 
near their sources to their mouths, are not associated with 
lakes, except perhaps such as are formed by the streams 
themselves in alluvial-filled valleys. 

A comparison of many streams will show that the differ- 
ences just referred to, depend on their age, or, more accur- 
ately, on the stage of development they have reached. 



54 RIVERS OF NORTH AMERICA 

Youne streams, or such as have not cut down their channels 
so as to produce a uniform gradient, are the ones suppHed 
in part by lakes, and are broken by places of rapid descent ; 
while older streams have removed the inequalities from their 
channels, and the lakes that may formerly have existed along 
their courses have been drained. It thus becomes evident 
that the degree to which variations in the hardness of the 
rocks influence the scenery of streams is greatest in youth 
and gradually beomes less and less, but seldom, if ever, 
entirely vanishes. 

All stages in development, from extreme youth to slug- 
gish old age, may be recognised in the streams of North 
America, and among the most marked characteristics of this 
slow change is the presence of cataracts and rapids in the 
courses of the streams which have not yet made a decided 
advance in their appointed tasks. 

Waterfalls, — Young streams are obliged to accept in- 
herited conditions of slope, and may discover that their 
courses are broken by places of deep descent, and rapids 
and cascades result. If, for example, a stream originates 
on a tableland, with an irregular surface, or is perhaps 
bounded by an escarpment, it will have an uneven channel, 
at least for a time, and be broken by places of steep descent. 
Again, streams coming into existence on the withdrawal of 
an ice-sheet, or the draining of a lake, will usually find in- 
equalities in their channels which will cause waterfalls. In 
such instances the conditions producing the falls are an in- 
heritance from pre-existing topographic conditions. As 
stream development progresses, however, the 'cascades re- 
sulting from inherited conditions disappear; but others due 



INEQUALITIES IN THE HARDNESS OF ROCKS 



55 



to inequalities in rock texture, to the more rapid rates at 
which a trunk stream may deepen its channel than its 
branches, to the loads sometimes deposited in sluggish 
streams by swifter 
tributaries, and still 
other causes, make 
their appearance. 

In illustration of 
the processes by 
which cascades origi- 
nate through the ac- 
tion of the streams 
themselves, it is evi- 
dent that when a 
stream flows across 
alternating hard and 
soft beds, the soft 
beds will be removed 
more easily than 
those of greater resist- 
ance, and when the 
streams leave a hard 
bed, a rapid, cascade, 

or waterfall may be Fig. 2. A Young Valley being Cut in Shale 
produced. The refer- ^^ck, Central New York. (After R. S. Tarr.) 

ences just made to a trunk stream cutting more rapidly 
than its tributaries, and the deposition of debris in a sluggish 
stream at the mouth of a high-grade tributary, need no 
special explanation. 

All of the conditions just referred to, including inherited 




56 RIVERS OF NORTH AMERICA 

inequalities of channel and the production of places of steep 
descent during stream development, pertain prnicipally to 
young streams. As a stream advances in its task of cutting 
down to baselevel and acquires the gradient best adapted 
to its work, inequalities in the slope of its channel disap- 
pear. Cataracts, of whatever character, are usually an index 
of immature stream development. The development of a 
stream progresses, however, from its mouth towards its 
source, and the feeding brooks of even well-developed river 
systems are young, and may have cascades, while the lower 
portions of the same drainage system may have reached 
perfect adjustment. The streams draining the southern 
Appalachians have been allowed to progress with the execu- 
tion of their tasks without serious interruption for a long 
period of time, all lakes and waterfalls resulting from in- 
herited conditions, and practically all cascades produced by 
irregularities in rock texture, have long since disappeared 
throughout their lower courses, but their head branches are 
still young and are yet being extended. On these young 
twigs of the drainage system cascades are common. An 
illustration of a cascade of this nature is presented in 
Plate IV. 

An exception to the rule that cascades are not developed 
in the courses of mature streams, is sometimes found in lime- 
stone regions where surface drainage enters underground 
channels. The breaking of the roof of a cavern may lead to 
the production of a cascade at any stage in the life of a 
stream, but the chances of such an accident, as it may be 
termed, become less and less as baselevel conditions are ap- 
proached. 



INEQUALITIES IN THE HARDNESS OF ROCKS 57 

In addition to the causes just considered, there are 
changes produced by movements in the earth's crust, as 
when the rocks are folded or faulted ; the birth and growth 
of glaciers; volcanic eruptions; the deposition of rock ma- 
terial from solution, as when a spring precipitates traver- 
tine, siliceous sinter, etc., in the course of a stream; the 
work of animals, as when beavers build their dams ; the 
stranding of driftwood so as to block drainage, etc., which 
may interrupt the even flow of water, and give origin to 
rapids, cascades, and waterfalls. 

The tens of thousands of waterfalls in North America 
may be arranged principally in two classes: those resulting 
from the excavation of alternating hard and soft layers, and 
occurring mostly on the head branches of well-developed 
streams, as in the southern Appalachians; and those due to 
previous glacial conditions. Of these two classes the second 
is by far the more numerous, and furnishes the most magnifi- 
cent examples of waterfall of all grades. 

As already stated, the northern half of North America 
was formerly covered by ice-sheets. Local centres of snow 
accumulation and of glaciers also existed on the Rocky and 
Cascade Mountains, and the Sierra Nevada, far to the south 
of the southern limit of the former continental glaciers. 
Throughout nearly all of the vast regions which were form- 
erly ice-covered, the melting of the glaciers left the- land 
encumbered with debris. The surface inherited by the post- 
glacial streams was essentially a new-land area, and the 
streams have not progressed far enough in their develop- 
ment to have removed the inequalities in their channels, 
and waterfalls are common. 



58 RIVERS OF NORTH AMERICA 

Illustrations of this class of cascades are furnished by 
those in the picturesque streams of the Catskills, Trenton 
Falls, the many beautiful cataracts near Ithaca, the well- 
known instances in Watkins Glen, and numberless others 
of the same general character in New York. Still more 
numerous instances might be cited in Canada, as, for 
example, the leap made by the water at the Falls of Mont- 
morenci, near Quebec, the Great Falls in Labrador. In 
fact, scarcely a stream can be ascended in all of the eastern 
and northern portion of the formerly glacier-covered region, 
without discovering that it has recently been turned from 
its former channel, or exhibits the characteristics of youth 
from mouth to source. 

On the headwater of the Mississippi, and about the upper 
Great Lakes, the drift sheet is thicker than farther eastward, 
and the streams have less frequently cut down to the hard 
rocks beneath, so as to develop cascades. It is only the 
stronger rivers in this more thoroughly drift-covered country 
that have progressed far enough with their recently added 
task, to lay bare the solid rock beneath the superficial 
covering. 

A far-reaching result of the disturbance produced in 
stream development by the glacial epoch is seen in the dis- 
tribution of water power produced by it. In New England, 
manufacturing industries were soon established after the 
coming of Europeans, and a decided impression made on 
the character of the people by this circumstance. South of 
the glacial boundary, water power was far less abundant, 
and mostly within the more inaccessible portions of the 
mountains; the development of manufacturing industries 



INEQUALITIES IN THE HARDNESS OF ROCKS 59 

was hence delayed, and attention given more largely to 
agriculture, for which climatic and other conditions were 
more favourable. When steam was introduced as a motive 
power, the waterfalls at the north declined in importance as 
sources of energy, but in this budding age of electricity they 
are again coming into demand. 

In all of the various phases of stream development and of 
the diversity in the relief of the land produced by stream 
erosion, thus far considered, a marked feature has been that 
changes are continually in progress. To this rule, the water- 
falls furnish no exception. They have their periods of 
growth and decline, and in many instances shift their 
positions, or migrate. 

In the case of waterfalls resulting from inherited topo- 
graphic conditions, they may spring into existence all at 
once, and at the very start be grander than ever after. 
Niagara, when it first leaped from the summit of the escarp- 
ment near the present site of Lewiston, was higher than at 
any subsequent period of its history.' 

When falls result from the wearing away of soft rocks so 
as to make adjacent hard beds prominent, there is usually 
at first a small difference in relief, producing a rapid, and 
then greater changes resulting in a cascade the height of 
which is increased by reason of the increased energy of the 
waters as they plunge into the pool below, but there comes 
a time when the stream channel below the fall can be low- 
ered no farther. The cascade, or waterfall, as we may 
choose to call it, then reaches its greatest development, or 

^ G. K. Gilbert, *' Niagara Falls and their History," in National Geographic 
Monographs, vol. i., pp. 203-236. American Book Co., 1895. 



6o 



RIVERS OF NORTH AMERICA 



its majority. But the lowering of the stream channel above 
the fall continues, and its height gradually decreases. Such 
a sequence in the life of a waterfall originating from unequal 
stream corrasion in soft and hard rocks, w^ould result when 
the fall did not migrate up stream but remained in one 
place. But most waterfalls are subject to a process of 
migration. 

TJie Migration of Waterfalls, — When a hard layer causing 
a waterfall is horizontal, or but slightly inclined, the escarp- 
ment over which the waters plunge recedes, owing, in most 
instances, to the removal of softer rocks beneath, by the 
friction of stones washed about by the swirling waters, and 
the fall migrates up stream, leaving a more or less canyon- 
like valley to mark the path along 
which it travelled.' Thus, below Ni- 
agara Falls there is a canyon about 
seven miles long, and approximately 
two hundred feet deep, which has 
been left by the migration of the 
cataract. 

If the hard bed cut through by a 
stream so as to produce a cascade has 
a sharp downward slope in the direc- 
tion opposite to the flow of the stream, 
the fall will become lower and lower 
as corrasion progresses, and when the 
hard layer is passed, the life of the 
cataract will come to an end. If, 
however, as may happen, the hard layer dips downstream, 
the rapid or fall produced will manifestly increase in magni- 




FiG. 3. Profile and Sec- 
tion at Middle of Horse- 
shoe Fall, Niagara, 
Showing Hard Lime- 
stone above Soft Shale, 
and Probable Depth of 
the Pool into which the 
Waters Plunge. Scale : 
I inch = 384 feet. (After 
G. K. Gilbert.) 



Plate IV. 




Fig. a. Fall on Black Creek near Gadsden, iVlabama. 

A young branch of Coosa River, at the south end of Lookout Mountain ; hard sandstone 

above shale. 




Fig. B. Echo River in Mammoth Cave, Kentucky. 
(Copyrighted photograph by H. C. Ganter.) 



INEQUALITIES IN THE HARDNESS OF ROCKS 6 1 

tude until the locality where the outcropping edge of the 
hard bed comes to the surface is cut through, and then a 
comparatively sudden lowering and adjustment of grade 
will follow. When the hard layer which a stream has to cut 
through is horizontal instead of being inclined either with 
or against the current, it presents a greater task to a stream 
cutting through it than in any other position, because the 
mass of rocks necessary for the stream to remove in order 
to reduce the grade of its channel is greater than if the 
bed is inclined. When the rocks are horizontal, the life of 
a cataract may be immensely prolonged. 

The only cases in which waterfalls produced by hard ad- 
jacent to soft rocks do not migrate, are when the hard layer 
is vertical. In such a case the change in the position is 
limited to the thickness of the resistant bed. In the Cas- 
cade Mountains there are numerous rapids and cascades due 
to vertical dikes of basalt. These dikes are harder than 
the adjacent rock, and cause inequalities in the beds of 
the streams crossing them, but the positions of the falls 
produced remain essentially the same throughout their lives. 

In the streams flowing eastward from the Appalachians, 
falls and rapids occur where they leave the hard crystalline 
rocks forming the Piedmont Plateau and enter the soft rocks 
of the Coastal Plain. As these falls recede up stream, they 
leave canyons to mark the paths they follow. 

Many of the falls in the drift-covered region of North 
America are due to the turning of streams from pre-glacial 
valleys in such a way as to cause them to flow over what 
were formerly divides or rocky spurs between adjacent 
streams and plunge into valleys. In some instances, also. 



62 RIVERS OF NORTH AMERICA 

they have cut through a covering of drift and been lowered 
upon harder rocks beneath. In either case falls may result. 
Cascades are also produced where these streams leave a 
region of hard rock and enter areas of drift which is more 
easily removed. 

Niagara Falls came into existence when a large lake, which 
formerly flooded both the Ontario and Erie basins, was 
lowered so as to be divided into two water bodies by a ridge 
trending east and west, formed by the summit of the Lewis- 
ton escarpment. 

The Falls of St. Anthony are due to the Mississippi 
having been turned from its pre-glacial course by deposits 
of drift, and made to flow over the surface of a compara- 
tively thin horizontal sheet of limestone resting on soft 
sandstone. A cataract about one hundred feet high was 
produced where the river left the edge of the limestone 
layer and plunged into a pre-glacial valley. From this 
escarpment the fall has receded about eight miles, leaving a 
steep-sided canyon, as in the case of Niagara. 

Shoshone Falls, Idaho, were produced by a hard sheet of 
trachyte which Snake River discovered as it sank its chan- 
nel in nearly horizontal layers of basalt. The falls have 
migrated up stream, leaving a narrow canyon as a record of 
the work already performed. 

Many beautiful cascades in the Rocky and Cascade 
Mountains, and others no less picturesque in the Sierra 
Nevada, are due to changes produced by a former period 
of glaciation. In some instances in the Sierra Nevada, large 
Alpine glaciers flowed down the main valleys, and blocked 
the streams in lateral gorges so as to cause them to cease 



' INEQUALITIES IN THE HARDNESS OE ROCKS 63 

corrading. The glaciers deepened the main valleys, how- 
ever, during the time the development of their tributary 
streams was arrested, and when the ice melted and water 
drainage was once more established, the branches of the 
main streams were compelled to descend steep precipices 
and in many instances form fine cascades, in order to reach 
the bottoms of the glacier-deepened main valleys. 

The ancient glaciers, to which so many references have 
been made, brought destruction in their paths as they ad- 
vanced and left fields of desolation as they retreated, but in 
many ways the beauty of the region they occupied was en- 
hanced by the changes they made. Our greatest debt to 
the vanished glaciers, so far as the revolutions they wrought 
appeal to our artistic sense, is for the tens of thousands of 
placid lakes they left strewn over the land, and the tens 
of thousands of leaping waterfalls which sprang into exist- 
ence on their retreat. The former are emblems of rest, the 
latter of ceaseless activity. 

Bluffs Bordering Aged Streams, — As previously stated, 
the influence of the unequal yielding of hard and soft rocks 
is most marked in the case of streams that are still young, 
and decreases as they advance in development, but seldom 
entirely disappears. 

Topography being largely the result of the action of 
streams, it follows that the various features in the relief 
of the land must be due, to a marked extent, to the un- 
equal waste of hard and soft rocks. The hard rocks stand 
as bluffs, ridges, and peaks, while the soft rocks are worn 
away more rapidly, and dells and valleys appear. The 
various stages from youth to old age, so characteristic of 



64' RIVERS OF NORTH AMERICA 

streams, find a counterpart in the general changes in form 
and expression experienced by the surfaces of land areas. 
A new land area may have a generally even surface, but as 
time passes, and topographic maturity is reached, it becomes 
roughened, and if the elevation has been great, and marked 
inequalities in the hardness of the rocks occur, exceedingly 
rugged topographic forms will be developed. When a land 
area has been long exposed, the inequalities of surface due 
to differential weathering gradually decrease, but except in 
the rare instances of nearly complete baselevelling, do not 
disappear. 

Throughout the courses of streams that have passed their 
periods of maturity, and even after having developed a 
gentle gradient characteristic of old age, their valley walls 
frequently retain evidences of the great topographic diver- 
sity that characterised them during youth. Rivers flowing 
through lands having in general all the characteristics of 
topographic old-age may yet be bordered in places by steep 
bluffs and overshadowed by towering precipices. 

The Highlands of the Hudson, where the river valley is 
narrow and bordered on each side by rugged mountains, in 
contrast with the wider portions above and below, where the 
bordering uplands are less precipitous, reveal the influence 
of hard rocks on the scenery of an ancient river. The 
picturesque ** coves** in the Southern Appalachians, as 
along the upper course of the Hiawassee, have been hol- 
lowed out in soft beds, and are surrounded by precipitous 
mountains of hard rock. 

Many bold headlands in the upper Mississippi valley are 
remnants of ancient eminences, rounded and worn by long 



INEQUALITIES IN THE HARDNESS OF ROCKS 65 

exposure, which in several instances rise directly from the 
border of the river that sweeps about them and has long 
since passed its period of youth. 

Much of the wonderfully impressive scenery of the 
Columbia is due to great bluffs of basalt which rise di- 
rectly from the river's brink, and on account of their 
hardness, in contrast with softer beds adjacent, remained 
prominent even after the river had cut down its channel to 
an even grade and become navigable. 

The same sequence of events was noted by the writer 
in many instances while ascending the Yukon. That noble 
river, although well adjusted to the various rock conditions 
it discovered as it deepened its channel, and flowing with 
such an even grade that it can be ascended by steamboats 
for over fifteen hundred miles, is bordered in places, as 
shown in Plate II., by magnificent bluffs of hard rock which 
intervene between long reaches where the valley is several 
miles broad, and has been excavated in softer beds. 

While the persistence of the topographic forms on the 
borders of river valleys is conspicuous, and accounts for 
many of the more prominent features adjacent to aged rivers, 
yet the lives of many streams have been so greatly prolonged 
that movements in the earth's crust have produced changes 
simulating those just considered. The rocks crossed by a 
great river may be upraised so as to form ridges, or even 
mountain ranges, athwart its course, and dam its waters or 
turn them aside. When such changes occur, however, with 
sufficient slowness to allow the river to deepen its channel 
as fast as the rocks rise, it will maintain its right of way, 

and excavate a gorge or canyon through the obstruction. 

5 



66 RIVERS OF NORTH AMERICA 

In such instances a portion of the stream will have the charac- 
teristics of youth, while adjacent portions above and below, 
whose development was unchecked, present all the features 
of old age. Rivers which maintain their right of way in 
the manner just cited, and carve gorges and canyons through 
newly elevated lands, have been termed antecedent rivers by 
Powell, in recognition of the fact that they are antecedent 
to the movement which causes the rocks to be elevated. 



CHAPTER IV 

MATERIAL CARRIED BY STREAMS IN SUSPEN- 
SION AND IN SOLUTION 

THE waters flowing from the land back to the sea, 
whence they came as vapour, carry material with 
them, as is well known, in two distinct ways: namely, in 
suspension and in solution. The debris carried in suspen- 
sion and rolled along the bottom, or the visible load, as it 
may be termed, sooner or later finds its way to the sea and 
forms stratified deposits. The material dissolved by the 
waters during their excursion through the air and over the 
land, or their invisible load, goes to increase the salinity of 
the sea and to supply marine plants and animals with sub- 
stances necessary for their growth. 

THE VISIBLE LOADS OF STREAMS 

The material transported mechanically by streams may be 
divided into two classes: 1st, the portion rolled and pushed 
along the bottom; and, 2d, the portion lifted well above 
the bottom and carried forward in suspension. The divid- 
ing plane between these two classes is indefinite, as much 
of the material moved along the bottom makes short up- 
ward excursions, and the fine particles normally carried 
forward in suspension from time to time rest on the bottom. 

67 



68 RIVERS OF NORTH AMERICA 

Bottom Load, — Concerning the manner in which the bot- 
tom load, as it may be termed, is moved, and the amount of 
such transportation in a given stream, but httle information 
is available. 

If we watch a clear stream supplied with sand or gravel of 
such size that the current has power to move it, it will be 
seen that the debris does not advance as a continuous sheet, 
but rather as a succession of wave-like forms. The action 
of the water-current in this respect is similar to the be- 
haviour of air-currents when moving over dry sand. The 
ripple-like ridges on the bottom of streams are frequently 
and probably always broad in reference to their height. 
The up-stream slope of each ridge is gentle and its down- 
stream border short and precipitous. Grains of sand are 
moved over the broad gently ascending surface and rolled 
down its steep down-stream margin. At the base of the 
steep border of each ripple-like sheet, there are secondary 
currents caused by the plunging of the water, and the 
particles forming the bottom are there disturbed and carried 
onward and the process repeated. When the material at 
the bottom is in excess of the transporting power of the 
stream, sedimentation takes place, and cross-stratified or 
current-bedded accumulations result ; but if the bottom cur- 
rent is under-loaded, the material is carried forward by being 
removed from the up-stream margin of a broad ripple-like 
sheet, and re-deposited on its steep down-stream margin. 

The process just described goes on at the bottom of clear 
streams, and illustrates the fact that such streams, contrary 
to what is sometimes stated, have power to corrade. It is 
only when their bottoms are swept clean of all grains of such 



MATERIAL CARRIED BY STREAMS 69 

sfze as are within the capacity of the stream to sweep away, 
that mechanical corrasion ceases. 

These statements concerning the bottom loads of streams 
may be said to be qualitative, inasmuch as measures of the 
amount of material thus transported are lacking. It is diffi- 
cult and at present seemingly impossible to ascertain how 
much material a large river is moving in the manner just 
considered. Bottom transportation will evidently vary with 
changes in conditions, being favoured by swiftness of cur- 
rent and the character of the debris available for transporta- 
tion. Variations probably also occur in reference to the 
amount of material a stream carries in suspension. If a 
stream is heavily charged with silt, the friction of flow will 
be increased, and as this friction is greatest in proportion to 
rate of flow, near the bottom, it is to be expected that bot- 
tom transportation will be checked while transportation in 
suspension is still actively progressing. It would seem, 
therefore, as if bottom transportation is favoured by de- 
crease of material in suspension ; or, other conditions being 
the sam.e, clear streams have a greater power to move bot- 
tom loads than muddy streams. I must confess, however, 
that this is theory rather than a deduction from observations 
and experiments, and the reader is invited to test the con- 
clusion for himself. 

It is evident that the principal conditions favouring bot- 
tom transportation are velocity and volume of water. As 
velocity increases with declivity, we should expect that 
high-grade streams would move proportionately heavier 
loads along their bottoms than low-grade streams, other 
conditions being the same. The ratio of bottom load to 



70 RIVERS OF NORTH AMERICA 

total transportation should be greater during floods than 
during low-water stages. Observation seems to confirm 
these conclusions. 

The proportion of bottom load to the amount of material 
carried by a stream in suspension is dependent largely on 
the character of the debris within the reach of the stream — 
that is, whether it is fine or coarse ; but in general it seems 
true, as just stated, that the bottom load in swift streams is 
greater in proportion to the amount of material in suspen- 
sion, than is the case in slower streams under similar second- 
ary conditions. 

Although the manner in which bottom loads are carried 
is not thoroughly understood, and the amount of such trans- 
portation difficult to determine, the fact remains that much 
of the energy of streams is consumed in rolling debris along 
their bottoms. In many measures of the rate at which 
streams are removing rock debris from their drainage basins, 
the quantity moved along their bottoms is not considered. 
For this reason most estimates of the rate at which land 
areas are being lowered by denudation require important 
modifications. 

Measures of Material in Suspension, — The methods em- 
ployed for ascertaining the amount of sediment carried by 
a stream in suspension are illustrated by the careful work 
done in this connection by Professor Forshey, during the 
survey of the Mississippi by the United States Topographic 
Engineer Corps.* Stations were selected near Carrollton, a 
short distance above New Orleans, one about three hundred 

' Humphreys and Abbot, Report upon the Physics and Hydraulics of the 
Mississippi River ^ p. 137, 186 1. 



MATERIAL CARRIED BY STREAMS 7 1 

feet from the east bank of the river, the next in mid-stream, 
and a third about four hundred feet from the west bank. 
The high-water depths at these stations were lOO, lOO, and 
40 feet respectively. Samples of water were collected daily 
at surface, mid-depth, and bottom at the first two stations; 
and at surface and bottom at the third station for a period 
of one year. During the succeeding year, the ratio between 
the sediment contained in the water at any one station and 
that contained in the entire cross-section of the river having 
been ascertained, one sample was taken each day from the 
surface at the station near the east bank. 

The samples from below the surface were secured by 
means of a small keg heavily weighted at the bottom and 
provided at each of its ends with a large valve opening up- 
ward. These valves allowed a free passage to the water 
while the keg was sinking to the required depth, but pre- 
vented its escape while being drawn up. When the keg 
reached the surface, the water contained in it was thor- 
oughly stirred and a bottle filled from it. The sediment 
contained in the water samples was subsequently filtered 
out and weighed after drying. 

From the tabulated results of the first year's work re- 
ferred to, it was found that the greatest amount of sediment 
was carried in June, during the annual high-water stage of 
the river, the weight of sediment then being -g-J^ of the 
weight of the river water containing it; the minimum was 
obtained during the low-water stage late in October, the 
ratio of weight of sediment to weight of water then being as 
I to 6383. The mean for the year was y^Vs"^ ^^ ^^^ ^^^ ^f 
sediment to 1808 tons of water. 



72 



RIVERS OF NORTH AMERICA 



A discussion of a still larger number of observations, 
made under the direction of Humphreys and Abbot, gave 
the ratio of i of sediment to 1500 of water by weight, and 
of about I to 2900 by volume. 

The variation in the amount of sediment with positions in 
the stream is indicated in the following table of the weekly 
means for the two months of highest and lowest water 
respectively : 

SEDIMENT IN THE MISSISSIPPI AT CARROLLTON 





FIRST POSITION. 


SECOND POSITION. 


THIRD POSITION. 


NUMBER OF WEEK. 


1 

3 
C/3 




6 




PQ 






S 





oi 


3 
CO 








First in June, 1851 

Second *' " " 

Third '' " " 

Fourth " " " . 


0.345 
0.456 
0.917 
0.498 


0.407 
0.507 
0.960 
0.570 


0.187 
0.510 
0.940 

0.557 


0.365 
0.477 
0.731 
0.528 


0.415 
0.515 

0.981 

0.597 


0.410 
0.517 
1. 105 
0.601 


0.285 
0.365 

0.666 
0.427 


0.3900.365 
0.4570.442 
1.0460.447 
0.5360.452 


Mean for June 


0.559 


O.61I 


0.548 


0.525 


0.627 


0.658 


0.436 


O.6O7JO.426 


First in October, 1851.. 
Second '' '' " . . 
Third '' 
Fourth" 


0.137 
0.120 
O.IOO 
0.068 


0.187 
0.169 
0.132 
0.096 


0.220 
0.170 
0.136 
0.106 


0.125 
0.109 
0.097 
0.059 


0.215 

0.193 
0.146 
O.I 15 


0.235 
0.220 

0.159 
0.116 


0.096:0.265 0.170 
0.107 0.235 0.092 
0.0890.195 0.071 
0.061 0.136 o.oSi 


Mean for October 


0.106 


0.146 


0.158 


0.096 


0.172 


0.182 


1 1 
0.0880.2080.104 



The figures denote the number of grammes of dry sediment contained in 600 
grammes of river water. 

Knowing the amount of water discharged annually by the 
Mississippi and the proportion of sediment contained in it, 
the amount of material carried by the stream each year in 



MATERIAL CARRIED BY STREAMS 73 

suspension may be readily computed. The mean annual 
discharge, as determined by the survey in charge of Hum- 
phreys and Abbot,* is 19,500,000,000,000 cubic feet, and the 
amount of soHd matter carried in suspension 812,500,000,- , 
000 pounds.^ The average specific gravity of this material 
is about 1.9; with this density, the sediment carried annually 
would occupy 6,718,694, 400 cubic feet, or sufficient to cover 
one square mile to the depth of 241 feet. 

In addition to the silt carried in suspension, it has been 
estimated by the engineers cited above, that the amount of 
sand and gravel rolled along the bottom and contributed 
each year to the filling of the Gulf of Mexico is about 
750,000,000 cubic feet ; making the total visible load carried 
by the river each year about 7,468,694,400 cubic feet, or 
sufficient to cover one square mile to a depth of 268 feet. 

The Mississippi has been more carefully studied than any 
other river in North America, but it is well known that 
other streams are doing a similar work. In many streams 
the proportion of material in suspension to the amount of 
water is greater than in the lower Mississippi ; while rivers 
might be selected, as, for example, the St. Lawrence, in 
which the percentage of sediment is much less. An inspec- 

' Report on the Mississippi River, p. 149. If I understand this portion of 
Humphreys and Abbot's report correctly, the above measures do not include 
the three outlet bayous, which leave the main river above New Orleans. It is 
stated on page 93 of the report, that, including these bayous, the annual 
discharge is 21,300,000,000,000 cubic feet of water. 

-Taking the specific gravity of water as i, the relative weight of coarse 
river-sand is 1.88; fine sand, 1.52 ; clay, 1.90; alluvial matter, from 1.92 to 
2.72. A cubic foot of water weighs 62.5 lbs. ; of coarse sand, 11 7. 5 lbs. ; fine 
sand, 95 lbs. ; clay, 118.75 lbs. ; alluvial matter, 120 to 170 lbs. ; silt, 103 
lbs. W. H. Wheeler, Tidal Rivers ^ p. 62. Published by Longmans, Green, 
«& Co., 1893. 



74 



RIVERS OF xXORTH AMERICA 



tion of the following table, compiled by C. C. Babb,' in 
which data concerning the amount of material that is being 
carried by several large rivers are presented, shows that the 
Mississippi, although commonly recognised as a muddy 
stream, holds a smaller percentage of silt in suspension than 
several other rivers with which it may be compared. 

DISCHARGE AND SEDIMENT OF LARGE RIVERS 





z 

< Ji 

Q 


MEAN ANNUAL DIS- 
CHARGE IN CUBIC 
FEET PER 
SECOND. 


SEDIMENT. 


RIVER. 


'c3 


Ratio of sedi- 
ment to water 
by weight. 


Height of col- 
umn, one sq. 
mile base. 
Feet. 




Potomac 

Mississippi 

Rio Grande 

Uruguay 

Rhone 


11,043 

1,244,000 

30,000 

150,000 

34,800 

27,100 

320,300 

1,100,000 

125,000 


20,160 

610,000 

1,700 

150,000 

65,850 

62,200 

315,200 

113,000 

475,000 


5,557,250 

406,250,000 

3,830,000 

14,782,500 

36,000,000 

67,000,000 

108,000,000 

54,000,000 

291,430,000 




3,575 

1,500 

291 

10,000 

1,775 
900 
2,8So 
2,050 
1,610 


4.0 

241.4 

2.8 

10.6 

31. 1 
59.0 
93.2 

38.8 
209.0 


.00433 
.00223 
.00116 
.00085 
.01075 
.01139 
.00354 
.00042 
.02005 


Po 


Danube 

Nile 


Irrawaddy 


Mean 


334,693 


201,468 


109,649,972 


i: 2,731 


76.65 


.00614 



Estimates similar to those given in the above table have 
been published by several geologists. / One series of these, 
probably as reliable as any, by Archibald Geikie,'* is here 
copied for the purpose in part of showing that the observa- 
tions now available concerning the work of streams are de- 
fective. Even the areas of hydrographic basins are stated 
differently by different writers, and with perhaps a few 

' Science, vol. xxi., p. 343, June, 1S93. 

^ Text-Book of Geoiogy, 2d edition, p. 428. Macmillan & Co., 1885. 



MATERIAL CARRIED BY STREAMS 



75 



exceptions the measures given of the annual discharge of 
large rivers, the amount of sediment they carry, etc., should 
be considered as subject to corrections. 

SEDIMENT OF RIVERS 



RIVER. 


AREA OF BASIN IN 
SQUARE MILES. 


ANNUAL DISCHARGE 

OF SEDIMENT IN 

CUBIC FEET. 


FRACTION OF FEET 

OF ROCK BY WHICH 

THE AREA DRAINED 

IS LOWERED IN 

ONE YEAR. 


Mississippi 


1,147,000 

143,000 

700,000 

25,000 

234,000 

30,000 


7,459,267,200 
6,368,077,440 

i7,52o,ooo,ooo(?) 
600,381,800 

1,253,738,600 
1,510,137,000 


1 


Ganges (UpDer) 


"5" "011X7 


Hoang Ho 


Rhone 


Danube 

Po 


^ST^ 







A brief discussion of the rate at which land areas are being 
lowered by the removal of material by streams will be given 
after the measures of mineral matter in solution have been 
considered. 

THE INVISIBLE LOADS OF STREAMS 

Water as it reaches the land as rain, snow, dew, etc., is 
never chemically pure, but contains both organic and in- 
organic matter in solution and dust particles in suspension. 
The substances most commonly occurring in solution in 
rain-water are shown by the following analysis of a sample 
collected near London, England ^ : 

.99 part in 1,000,000 of water. 

.22 

.50 ** 

.07 



Organic carbon 

Organic nitrogen 

Ammonia 

Nitrogen as nitrates and nitrites. 



Chlorine 6. 30 parts in 

Total solids 39-50 " 

* Quoted by W. P. Mason, Water Supply, p. 204. John Wiley & Sons, 1896. 



76 RIVERS OF NORTH AMERICA 

Examinations for chlorine in water samples representing 
the average condition of the rain-water at Troy, New York, 
for one year, gave a mean of 1.64 parts in a million. That 
is, each million pounds of rain-water contained 1.64 pounds 
of chlorine in solution.' 

The impurities in rain-water vary in character and amount 
in different localities. In general they are greatest near 
cities, and least in the open country at a distance from vol- 
canoes, gas springs, etc. They also vary with climatic con- 
ditions, being greatest in arid and least in humid regions, 
and greater in dry than in wet seasons. The amount of 
common salt is large near the sea, and normally decreases 
inland, but probably reaches a maximum in the neighbour- 
hood of saline lakes and over salt deserts. Rain-water, 
then, comes to the earth with its solvent power increased 
by the presence of various substances washed out of the air, 
but its ability to take up mineral matter in solution is greatly 
increased as it flows over the land. 

The soil usually contains organic matter which is easily 
dissolved. The most common substances thus added to the 
water, which enhance its chemical activity, are carbonic acid 
or carbon dioxide (COg), and a large group of organic acids, 
known as the humus acids; these, however, are unstable, 
and soon change to carbon dioxide. The organic acids are 
derived mainly from the decay of vegetation, but in part 
are of animal origin.'^ 

The percentage of the organic acids taken in solution by 

^ W. p. Mason, Water Supply, p. 205. 

* A. A. Julien, " On the Geological Action of the Humus Acids." in Amer- 
ican Association for the Advancement of Science, Proceedings, vol. xxviii., pp. 
311-410, 1879. 



MATERIAL CARRIED BY STREAMS J 7 

a given quantity of water percolating through the soil varies 
with different localities, being greatest when decaying 
vegetation is most abundant and where the temperature is 
high. In all portions of the earth's surface, however, the 
water, on coming in contact with the soil or with solid rocks, 
has the power to dissolve portions of them. The water 
which runs over the surface and is gathered quickly into 
streams has less opportunity to take up mineral matter in 
solution than that which percolates through the soil and in 
many instances descends into the hard rocks beneath and 
comes to the surface again as springs. The water flowing 
quickly over the surface and that following more or less ex- 
tensive underground courses are commingled in the streams, 
and send their combined tribute of dissolved matter to the 
sea. Chemical denudation thus assists the mechanical ac- 
tion of flowing water in lowering the land, and is an import- 
ant factor in the process. 

The rate at which rocks are dissolved varies not only with 
the rain-fall, with the amount of organic acids in surface and 
subterranean water, and with temperature, but is influenced 
especially by the nature of the rocks in various regions. The 
solution of mineral matter in general is greater, other con- 
ditions remaining the same, the higher the temperature, 
although this does not apply to limestone, and is greatest 
where the rocks are composed of easily soluble minerals. 
For these reasons the chemical composition of river-water 
varies, but the departure from a mean, as shown by a large 
number of analyses, is less than might at first be expected. 

By the time the surface waters have united to form rills 
they contain sufificient mineral and organic matter to give 



78 RIVERS OF NORTH AMERICA 

them a complex chemical composition. Throughout their 
journeys to the ocean, as they form brooks, creeks and 
rivers, and especially when travelling underground, they be- 
come more and more highly charged with dissolved mineral 
matter. The longer the waters are in contact with soil and 
rocks, and with the finely divided material held by them in 
suspension, temperature conditions, etc., remaining the 
same, the more highly charged they become with substances 
in solution. Evaporation also tends to concentration, but 
this process, particularly in humid regions, is more or less 
completely counteracted by direct precipitation. 

River-waters, filtered of all material in suspension, and 
evaporated to dryness, leave a solid residue, which is the 
principal portion (the more volatile substances escaping) of 
the foreign matter previously held in solution. These 
waters are fresh in the every-day use of the term, but in 
fact owe their agreeable taste and, to a certain extent, their 
health-giving qualities, to the mineral salts and gases con- 
tained in them. Irx Table A, analyses are given of the 
waters of a number of American rivers, which show that the 
principal substances in solution are calcium and carbonic 
acid, probably combined as calcium bicarbonate. In some 
instances, however, as in the case of Jordan River, Utah, 
calcium sulphate is in excess of all other salts. 

From a large number of analyses of water samples ob- 
tained from the rivers of Canada and the United States, it 
has been found that the average amount of total solids in 
solution is o. 1 5044 part in a thousand by weight ; of this 
material, 0.056416 part in a thousand is calcium carbonate. 
In a table of forty-eight analyses of European river-waters 



MATERIAL CARRIED BY STREAMS 



79 



given by Bischof,' the average of total solids in solution is 
0.2127, and the average of calcium carbonate 0. 1139 part 
per thousand. From the analyses of thirty-six European 
river-waters, published by Roth,'^ including some of those 
tabulated by Bischof, the average of total solids is 0.2033, 
and of calcium carbonate 0.09598 part per thousand. 

In both American and European river-waters, so far as 
can be determined from the data in hand, the average of 
total solids is 0.1888, and of calcium carbonate 0.088765 
part per thousand. These figures may be assumed to repre- 
sent the average of the solids in solution in the waters of 
normal rivers. It will be noticed that the average for cal- 
cium carbonate is nearly one-half the average for total solids. 

Knowing the annual discharge of a river and the percent- 
age of mineral matter carried in solution, we can ascertain 
the amount of dissolved matter that the river contributes 
annually to the ocean, or enclosed lake into which it flows. 
To one unfamiliar with studies of this nature, the amount 
of rock-forming material thus annually transported by a 
large river in an invisible state is astonishing. The follow- 
ing table, showing the total amounts of solids in solution 
carried by certain rivers, has been compiled from various 



sources : 

Rhine. 



5,816,805 tons per year. 
8,290,464 



Rhone 

Danube 22,521,434 

Thames 613,930 

Nile 16,950,000 

Croton 66,795 

Hudson 438,000 

Mississippi 112,832.171 

Chemical Geology, vol. i., pp. 76, 77. English edition, London, 1854. 
'■ Allgemein und chemische Geologic, vol. i., pp. 456, 457. Berlin, 1879. 



8o RIVERS OF NORTH AMERICA 

The fact that streams transport great quantities of dissolved 
mineral matter, derived from the rocks in the basins they 
drain, may be shown by computing the numbers of tons of 
material in solution in a cubic mile of river-water. This 
has been done by John Murray,' and the result, based on 
the average composition of the waters of nineteen of the 
principal rivers of the world, is given below: 

MATERIAL IN SOLUTION IN ONE CUBIC MILE OF AVERAGE 

RIVER-WATER ^ 

CONSTITUENTS. TONS IN CUBIC MILE. 

Calcium carbonate (CaCOg) 326,710 

Magnesium carbonate (MgCOg) 112,870 

Calcium phosphate (CagPgOg) 2,913 

Calcium sulphate (CaSO^) 34,30i 

Sodium sulphate (NagSO^) 31,805 

Potassium sulphate (K2SO4) 20,358 

Sodium nitrate (NaNOg) 26,800 

Sodium chloride (NaCl) 16,657 

Lithium chloride (LiCl) 2,462 

Ammonium chloride (NH4CI) 1,030 

Silica (Si02) 74,577 

Ferric oxide (FegOg) , 13,006 

Alumina (AlgOg) I4,3I5 

, Manganese oxide (MngOg) 5,703 

I Organic matter 79,020 

I Total dissolved matter 762,587 

It has also been computed by Murray, and published in 
the article just cited, that the volume of water flowing to 
the sea in one year, including all the land areas of the earth, 
is about 6524 cubic miles. From the average chemical com- 
position of river-water, it follows that about 4,975,117,588 
tons of mineral matter in solution are being removed annu- 

^ Scottish Geographical Magazine^ vol. iii., p. 76, 1887. 

' Acids and bases combined according to the principles indicated by Bunsen. 



MATERIAL CARRIED BY STREAMS 8 1 

ally from the land area of the earth. This process of 
removing material of the land in solution has, very properly, 
been termed clicmical demidation. 

It is instructive to follow the history of the material 
carried in solution by rivers, and to see what changes occur, 
especially in inland seas where ordinary river-waters are 
concentrated by evaporation, and in many instances the salts 
they contain precipitated in a crystalline form, and to ex- 
tend such studies to the ocean. Another fruitful line of 
investigation in this connection is the manner in which 
mineral matter in solution is eliminated. This occurs in 
part, as just mentioned, by chemical precipitation, but is 
effected to an equally great extent, and from dilute solu- 
tions, through the action of plant and animal life. These 
interesting studies, however, lie beyond the scope of our 
present thesis. 

RATE OF LAND DEGRADATION 

Measures of the amount of material carried by streams 
both mechanically and in solution furnish a means of ap- 
proximately determining the rate at which the surface of 
the land is being degraded. 

Mechanical Degradation. — As shown in the table on page 
74, the amount of silt carried annually by the Mississippi, if 
taken uniformly from the area it drains, would lower it j^tt 
of a foot. That is, considering only the material carried in 
suspension, the basin is now being lowered at the rate of 
one foot in 5376 years. If we take into account also the 
material rolled along the bottom, computed to be 750,000,- 



82 RIVERS OF NORTH AMERICA 

ooo cubic feet per year/ we find that the basin is being^ 
lowered at the rate of one foot in 4638 years. 

Studies of the Potomac River, conducted by the United 
States Geological Survey, have determined the fact con- 
cerning that river presented in the table on page 74. As- 
suming that one cubic foot of the silt carried by the Potomac 
weighs 100 pounds, the average annual amount transported 
would cover one square mile to a depth of 3.98 feet. If 
this amount should be taken uniformly from all parts of the 
area drained, it would be lowered 0.0043 of an inch, or ^7^5- 
of a foot. In other words, the Potomac is lowering its 
hydrographic basin at the rate of one foot in 2772 years. 
Other similar estimates are given in the table on page 75, 
which, if approximately correct, might be taken as indicat- 
ing the work that the rivers of the world are doing. It is 
probable, however, that except in the case of the Mississippi, 
in the table compiled by Geikie, the bottom loads of the 
rivers are not included. I am also inclined to doubt the ac- 
curacy of some of the other measures referred to. The 
average for the nine rivers tabulated is one foot of denuda- 
tion in about 9000 years. 

Chemical Degradation. — The importance of the slowly 
acting and invisible process by which the surface of the land 
is being lowered by solution, has only recently been recog- 
nised. The earliest definite discussion of the rate of chemi- 
cal degradation now in progress, so far as I am aware, is in 
a series of three papers by T. Mellard Reade.' In these 

• A comparatively slight discrepancy comes in here, since the specific gravity 
of the bottom load and of the silt in suspension is not the same. 

^ Republished with the title, Chemical Denudation in Relation to Geological 
Time. Daniel Dogue, London, 1879. 



MATERIAL CARRIED BY STREAMS 83 

instructive essays it is estimated that the amount of material 
removed in a century by the streams of England and Wales 
in solution, if spread evenly over the land from which it is 
derived, would have a thickness of .0077 of a foot. That is, 
it will take 12,987 years to denude the surface of England 
and Wales of one foot of solid matter by the process here 
considered, under the supposition that the material is taken 
evenly from all parts of the surface. 

The Mississippi, as previously stated, carries annually 
about 112,832,171 tons of mineral matter in solution. This 
amount of material may be considered as about equivalent 
to 1,350,000,000 cubic feet of limestone, and if spread evenly 
over the Mississippi basin would cover it to the depth of 
about ^y-J-o-o ^^ ^^^ iooX. ; or, in other words, chemical 
degradation is lowering that area at the rate of one foot in 
25,000 years.' 

Rate of Both Mechanical and Chemical Degi'adation. — The 
best and in fact the only approximately reliable measures 
we have of the rate of the combined mechanical and chemi- 
cal degradation by the rivers of North America, is in the 
case of the Mississippi. On account of the large size of the 
drainage area of that river and the variety of rocks forming 
its surface, as well as the diversity of climate included within 
its border, it may be reasonably assumed to represent about 
the average rate of degradation which is being performed by 
rivers in general. 

^ The material in solution is taken in part from the surface and in part from 
below the surface. While an estimate of the average lowering of the surface 
by degradation during a single year need not perhaps include the material 
removed in solution from below the surface, yet this should certainly be taken 
into account in estimates of average degradation. 



<S4 RIVERS OF NORTH AMERICA 

As we have seen, the Mississippi is lowering its basin by 
the process of mechanical degradation at the rate of one 
foot in 4638 years, and by solution at the rate of one foot 
j in 25,000 years. The average rate of general degradation 
is therefore about one foot in 3912, or, in round numbers, 
4000 years. 

This estimate of the rate at which the Mississippi is lower- 
ing its drainage basin is somewhat greater than is given in 
several text-books of geology, but in these the amount of 
material carried in solution and the bottom load of the 
river do not seem to have been taken into account. An 
inspection of the tables on pages 74 and 75, shows that the 
Mississippi is removing material from the land at a less rate 
than is the case with several other rivers, and at even a less 
rate than the average of the rivers tabulated. 

UNDERGROUND STREAMS 

The water which finds its way below the surface of the 
land for the most part percolates through the rocks without 
forming definite streams. When the rocks are readily 
soluble, however, as when limestone is present in thick 
layers, underground channels are frequently dissolved out, 
and subterranean streams occur of sufficient size to be 
classed as rivers. During the underground flow of water, 
whether percolating through porous rocks or forming streams 
in caverns, it is brought into contact with the material 
forming the earth's crust, thus facilitating solution. One 
marked difference between surface and subterranean streams 
is that the former perform the task of eroding the land 



MATERIAL CARRIED BY STREAMS 85 

mainly by mechanical means, while the latter carry on a 
similar work principally by solution. Underground streams, 
in part, occupy ready-made galleries or caverns, like the open- 
ing along fractures and faults, or the tunnels in lava streams 
formed by the flowing out of the still molten parts after 
a crust formed. More frequently, however, subterranean 
streams make passageways for themselves. This process 
is analogous to the excavation of the valleys by surface 
streams. We may carry this analogy a step farther, and 
say that the underground streams flowing through pre- 
viously made channels are subterranean consequent-streams, 
and those which dissolve out their own galleries are sub- 
terranean subsequent-streams. But there is little, if any, 
advantage in such a nomenclature. 

The conditions most favourable for the beginning and 
growth of subterranean streams are that the rocks should be 
of comparatively easy solubility, and also in thick, nearly 
horizontal, and unbroken layers, situated at a greater eleva- 
tion than the adjacent surface valleys. The rocks most 
easily dissolved by cold water, and the ones, too, which 
frequently occur in thick, nearly horizontal layers, are the 
limestones. For this reason most caverns are in such beds. 
If the soluble layer has a bed of less soluble rock in contact 
with it both above and below, the conditions for the pro- 
duction of large caverns are still more favourable. 

The advantage of having the soluble layer elevated above 
adjacent surface drainage is that the water flowing through 
the galleries opened in it may readily escape and flow 
rapidly. A roof of rock which does not yield readily to the 
solvent action of percolating water admits of the dissolving 



86 RIVERS OF NORTH AMERICA 

out of broad galleries beneath and decreases the liability of 
their roofs to fall. A roof to a natural cavern is as essential 
as a roof to a mine. 

Subterranean, like surface streams are dependent on rain- 
fall for their water supply. They are also influenced by 
other climatic conditions. A humid climate not only in- 
sures an abundant water supply, but favours the growth of 
vegetation, which contributes organic acids to the descend- 
ing waters, thus increasing their power to dissolve mineral 
substances. A dry climate not only decreases the water 
supply of subterranean streams, but, as will be shown later, 
favours the precipitation of mineral matter, principally 
calcium carbonate, in pre-existing rock-openings. 

The temperature element of climate also plays a part in 
the history of underground drainage. A warm climate, if 
humid, insures a luxuriant vegetation, and hence an abund- 
ant supply of organic acids, while a dry climate has a reverse 
influence. A cold climate, by leading to the freezing of the 
water in soils, checks percolation, and in other ways exerts 
an unfavourable influence on the tendencies of sub-surface 
waters to enlarge the galleries they flow through. 

Underground passages in many instances owe their incep- 
tion to joints and fractures of small width which are enlarged 
by solution. The percolating of water through porous 
rocks, however, on account of inequalities in rock texture 
or composition, might be freer along certain courses than 
along others, and thus lead to unequal waste by solution, 
and to the making of cavities and galleries. When once a 
beginning is made, from whatever cause, the flowing waters 
tend to enlarge the galleries they pass through by dissolv- 



MATERIAL CARRIED BY STREAMS 8/ 

ing their walls. The water which finds its way through 
such galleries eventually emerges as springs, or again 
reaches the surface by percolation. The springs supplied 
from caverns usually come to the light in the sides or bot- 
toms of valleys, and contribute their water to the surface 
drainage; but not infrequently they emerge at the bottoms 
of lakes, or even beneath the sea. 

Subterranean, like surface streams remove rock material 
both by mechanical and chemical means, but the relative 
importance of the two processes is reversed. Caverns are 
enlarged principally by solution, but mechanical wear is 
frequently and sometimes an important part of the process. 
The flow of water through small or tortuous passages, the 
beginning of caverns, is probably sluggish in most instances, 
thus favouring solution, but retarding mechanical corrasion. 
Water, in descending to small underground passages, mostly 
percolates through soil or rock debris, and is thus filtered. 
This, again, is unfavourable for mechanical abrasion, since 
solid particles, the tools with which flowing water performs 
the greater part of its mechanical work, are removed. As 
underground galleries become larger, and especially when 
the surface of the country is lowered by denudation, open- 
ings in their roofs are frequently made, into which surface 
streams plunge with all of their freight of material in sus- 
pension. The waters in the underground courses of such 
streams are more or less heavily charged with sediment, 
and mechanical corrasion assists in the enlargement of the 
passageways they flow through. The rocks removed in 
order to make subterranean galleries frequently contain 
sand grains, chert nodules, silicified fossils, etc., of difii- 



88 RIVERS OF NORTH AMERICA 

cult solubility. Such bodies are separated by the re- 
moval of the more soluble material enclosing them, and, 
being swept along by the streams, assist the debris 
brought from above in abrading the rocks with which the 
waters are brought in contact. The behaviour of under- 
ground streams is thus seen to be similar in many ways to 
surface streams. As will appear as we proceed, cavern 
streams not only remove rock material, but also make both 
mechanical and chemical deposits, thus still further bearing 
out their similarity to ordinary brooks and rivers. 

The features of subterranean streams just enumerated, 
and of the results they bring about, are illustrated in the 
well-known instance of Mammoth Cave, Kentucky. The 
surface rocks in that region are mainly horizontally bedded 
limestones, about 350 feet thick. Above the limestone 
there is a sheet of sandstone, which forms the roof of the 
higher series of galleries. The ground above the cavern, 
over an area of thousands of acres, is elevated from 300 to 
350 feet above the adjacent valley of Green River, and is 
mostly without surface streams. The rain-water is largely 
absorbed by the deep residual soil left by the solution of 
hundreds of feet of limestone, and percolates slowly down- 
ward to the galleries beneath. During summer, all of the 
water entering the cavern probably gains access in this man- 
ner, but during heavy rains, surface rills and brooks are 
formed which plunge into openings or *' sink-holes " and 
enter the underground galleries directly as heavily silt- 
laden streams. After rains, as the writer has observed, 
the streams within the cavern are muddy, flow swiftly in 
many places, and are actively engaged in mechanical as well 



MATER/AL CARRIED BY STREAMS 89 

as chemical corrasion. The manner in which Echo River, 
the largest stream in Mammoth Cave, is now enlarging its 
channel is much the same as in the case of many surface 
streams, except that, owing to the irregularities and espe- 
cially the numerous constrictions in the passages it follows, 
its waters, when in flood, come in contact with the roofs of 
the galleries in some localities, and corrasion occurs above as 
well as at the bottom and sides. Like many surface streams, 
Echo River has its rapids, cascades, and quiet reaches, and 
in places, also, is ponded and forms lakelets. A photo- 
graph of this remarkable river is reproduced in Plate IV. 
Applying the criteria to be described later, by means of 
which young surface streams may be distinguished from 
mature or well-adjusted streams, to this example of subterra- 
nean drainage, we find it to be but imperfectly adjusted to its 
environment, and, therefore, it may be designated as an im- 
mature subterranean river. It has not deepened its channel 
throughout to the level of Green River into which it dis- 
charges, and which determines the baselevel of subterranean 
drainage in the rocks forming the adjacent uplands. As is 
the rule with surface streams, the adjustment of Echo River 
to baselevel has progressed most rapidly in its lower course. 
Near where it comes to the light as a huge spring on the 
border of Green River, it is about on a level with that 
stream. Farther within the cavern, it has many high-grade 
reaches, and is fed by torrent-like tributaries. A farther 
advance in its life history, the present local baselevel being 
maintained, should be characterised by the development of 
features analogous to those of a broad valley. A great 
gallery, corresponding with the valley of a surface stream. 



90 RIVERS OF NORTH AMERICA 

should be formed with its floor about on a level with the 
adjacent portion of Green River. 

The lowering of Green River has led to the deepening 
of the channels of tributary underground streams, and the 
abandonment, as avenues of drainage, of many galleries 
that were formerly waterways. The occurrence of one 
series of galleries above another, or the origin of the several 
stories in the cavern-house, as Mammoth Cave may be 
termed, can be accounted for, in part at least, on the prin- 
ciple just referred to: the highest series of galleries having 
been formed at a time when Green River flowed at the 
level of their place of discharge, and each lower series dis- 
solved out during subsequent stages in the deepening of 
master rivers. 

A different explanation from that just suggested has been 
advanced by N. S. Shaler,^ who states that the floors of the 
various stories in the great cavern are formed of resistant 
layers, each of w^hich gave a lateral direction to the flowing 
water, until an opening was found leading to the next layer 
of limestone below, when the process of lateral excavation 
was repeated. Hard layers, or layers less pervious than 
those above and below, have had an important influence in 
the development of the cavern, but rather, it seems to me, 
in the direction of modifying the action of the streams in 
response to a lowering of the place of discharge, than in 
furnishing the main control. The adjustment of under- 
ground streams to rock texture is analogous to the adjust- 
ment of surface streams to the conditions furnished by hard 
and soft rocks. 

^ Aspects of the Earthy p. 109. Scribner's Sons, 1889. 



MATERIAL CARRIED BY STREAMS 9 1 

A surface stream, as the reader has learned, when carry- 
ing debris but not overloaded, corrades its channels; but if 
its velocity is checked, will deposit a part or all of its load. 
These same features characterise subterranean streams. 
When subterranean waters run swiftly they flow over bare 
rock, but when their velocities are checked, sediment is de- 
posited. Many of the galleries in Mammoth Cave and 
other similar caverns have long since been abandoned as 
avenues of drainage, and are deeply filled with what is 
termed ** cave earth." In part, this material has been 
brought into the cavern by streams from the surface, but to 
some extent, certainly, it is the residue left by the solution 
of limestone. Ordinary grey limestone contains about one 
per cent, of insoluble material which remains when the 
calcium carbonate is removed, and forms a reddish clay. 
The sedimentary deposits in caverns are frequently terraced ; 
showing that the streams after dropping their burden of silt 
have been able to again resume the work of transportation 
and to cut channels through it. Conditions favouring sedi- 
mentation are frequently brought about during high-water 
stages when the water in certain galleries is ponded, owing 
to constriction lower down their course. Subsequently, 
during low-water stages, the streams are not held in check, 
and resume the task of deepening their channels. 

In dwelling on the similarity between the mechanical 
action of surface and of sub-surface streams, I do not wish 
to be understood as advocating the view that caverns are 
enlarged principally by the friction of the debris in the 
water flowing through them ; the main agency in their 
growth in most, and possibly all, instances is solution. 



92 RIVERS OF NORTH AMERICA 

There is a process in the chemical action of subterranean 
streams analogous to the manner in which they mechanically 
corrade and deposit alternately. When the waters are 
abundant and.flow freely, they carry away all of the material 
contributed to them in solution, and passageways are en- 
larged. When, however, the water supply is greatly di- 
minished, and water falls from the cavern roofs mainly in 
drops, or descends their sides and flows over their floors 
in thin sheets, evaporation leads to an increase in the per- 
centage of mineral matter in solution and to the precipitation 
of certain salts. 

In addition to the evaporation, when the mineral-charged 
waters fall in drops, or are spread out in thin sheets, and of 
more importance in the history of caverns, is the fact that 
these conditions lead to the escape of carbonic acid. Lime 
(calcium carbonate), the most abundant substance dissolved 
by both surface and cavern waters, is held in solution owing 
to the presence of carbonic acid, and is precipitated when it is 
removed.* The calcium carbonate precipitated on the roofs 
of caverns frequently takes pendent or icicle-like forms, 
termed stalactites. After the water falls to the floor of the 
caverns precipitation is continued, and sheets and pillars of 
calcium carbonate, termed stalagmites, are formed. 

Calcium carbonate is by far the most abundant, and, in 
fact, in most caverns, seems to be the only precipitate 
thrown down. The roofs of caverns, however, are some- 

' Pure water when cold dissolves calcium carbonate in the proportion of one 
part of the salt to 10,800 parts of water, and 8875 parts if the water is boiling 
(Fresenius) ; but if charged with carbon dioxide cold water will dissolve one 
part in about 1000 : the calcium when in solution is in the condition of a 
bicarbonate. 



MATERIAL CARRIED BY STREAMS 93 

times beautified by rosettes and star-like brilliants of gypsum 
(calcium sulphate) and by other and more soluble salts 
which appear as crystalline efflorescences. 

It is this process of lining caverns with crystalline in- 
crustations, the forming of pendants of many shapes and 
tints from their roofs, as well as of no less beautiful and 
frequently grotesque stalagmite columns, that gives to those 
silent galleries of the nether world much of their fascination 
and beauty. One of the very finest examples of subter- 
ranean galleries, partially filled by calcareous precipitates 
from percolating waters, is furnished by the beautiful Luray 
Cavern, Virginia. Other remarkable illustrations of the 
same occurrence may be seen in the justly celebrated 
Wyandotte Cavern, Indiana. 

The process of infiltration, just noticed, if allowed to 
continue, will, in time, fill the cavern previously excavated 
when the water supply was abundant. In many caverns 
galleries occur which have been nearly closed by this 
method, and in other instances it is evident that the filling 
has been completed. Checks in the process of refilling may 
evidently occur from an increase in water supply or from its 
nearly complete cessation. Surface changes, such as the 
removal of vegetation, would also influence the conditions 
favouring enlargement, or refilling, in the caverns beneath. 

Many peculiarities in the surface features, particularly of 
limestone regions where caverns occur, have an intimate 
relation to the galleries beneath. The holes through which 
surface water descends are enlarged by solution or by the 
falling of portions of the roofs of the caves, and become 
basins termed *' sink-holes." Should the openings in the 



94 RIVERS OF NORTH AMERICA 

bottom of these depressions become filled, lakelets may be 
formed. Such basins and lakelets are a characteristic feature 
of many limestone regions, as, for example, in the Great 
Appalachian valley. Western Kentucky, and much of Ten- 
nessee. Some of the basins near Mammoth Cave, caused 
by the solution of the rocks beneath and the falling of the 
roofs of caverns, occupy an area of about 2000 acres. ^ 

The subsidence of the roofs of caverns also gives origin 
to trench-like depressions in the surface above, which be- 
come avenues of drainage. The course of Green River near 
Mammoth Cave, is through a depression of this character 
which has been modified by surface erosion. When portions 
of a cavern roof fall, leaving other portions in position, 
natural bridges and tunnels result. The most striking 
example of this nature is the justly famous Natural Bridge 
of Virginia. 

Another but minor influence of subterranean drainage 
on the relief of the region above, is brought about when the 
caverns by conducting away the surface waters decrease 
the rate of surface erosion. When this occurs and the 
lowering of the adjacent land continues, the rocks traversed 
by galleries are left in relief and form a mound or perhaps 
a series of hills, while the surrounding country, without 
sub-drainage, sinks into valleys. An illustration of this is 
furnished by the low hills in which Luray Cavern is located. 

Subterranean drainage goes on with the greatest freedom 
and produces the most conspicuous results above the level 
of adjacent surface streams. This assertion needs, perhaps, 

' Hovey and Call, The Mammoth Cave of Kentucky^ p. 4. Morton & Co., 
Louisville, 1897. 



MATERIAL CARRIED BY STREAMS 95 

to be qualified, as only such caverns as are above the level 
of neighbouring valleys are ordinarily open to inspection, 
while the size and extent of the galleries below the level of 
surface drainage can only be judged from indirect evidence. 
The flow of water through caverns above the level of the 
surface streams into which they discharge, except in part 
during high-water stages, is due directly to gravity — that is, 
the waters flow as through ordinary open channels; but the 
flow through lower galleries is produced by hydraulic pres- 
sure, and is similar to the movement of water through pipes, 
as in the case of the water-mains of a city/ There is a 
marked difference in these two methods in reference espe- 
cially to mechanical corrasion. The work of subterranean 
streams in galleries not completely filled is, in most in- 
stances, mainly in the direction of solution, but is aided also 
by the friction of particles in suspension or rolled along the 
bottom. In such instances, also, the conditions are nor- 
mally unfavourable for sedimentation. When water is 
forced through galleries by hydraulic pressure, they are 
completely filled up to the level of the surface of the re- 
servoir, friction is greatly increased, and the rate of flow is 
normally not rapid. This is an incomplete statement, to be 

' The possible influence of heat in causing water to flow through subterranean 
galleries has recently been discussed by F. W. Crosby and W. O. Crosby 
{Technology Quarterly, vol. ix., pp. 6-23, Boston, 1896) in connection with a 
study of what are known as the Sea Mills of Cephalonia, in Greece. In this 
instance fresh water at the ordinary surface temperature enters openings at 
sea-level and is supposed to emerge below sea-level and against the pressure 
of the denser sea-water. In explanation of this exceptional occurrence, it is 
suggested that the descending water traverses a more or less U-shaped system 
of galleries, and is heated in the ascending portion of its course and thus rend- 
ered lighter than the inflowing and colder water. 



96 RIVERS OF NORTH AMERICA 

sure, but sufficient, I think, to show that below surface 
drainage the conditions are less favourable for cavern-mak- 
ing than in similar rocks situated at higher levels. In 
galleries below surface drainage, it may be reasonably pre- 
sumed, mechanical corrasion is retarded and the conditions 
favouring sedimentation are augmented. 

Deep within the earth's crust the conditions differ from 
those near the surface. The tendency of pressure to 
close cavities increases with depth. The limit below which 
openings of such size as to be classed as caverns can 
exist, must be at a very moderate depth, — possibly not 
more than a few thousand feet. With increasing depth, 
also, there is a progressive rise of temperature (so far within 
the earth as man has ever penetrated), which exerts a 
marked influence on the solvent power of water. A rise 
of temperature, at least until excessive heat is reached, in- 
creases the solvent power of water for all common minerals 
except calcium carbonate. With the increase of temperature 
and of pressure, in subterranean waters, there is an increase 
in the variety and abundance of mineral substances which 
are taken into solution, and also increased chemical reac- 
tions, some of which lead to precipitation and to the filling 
of openings. These and still other reasons favour the belief 
that caverns, and consequently underground streams, do 
not exist below an extremely superficial portion of the 
earth's crust. ^ 

^ To the books of reference concerning American caverns already mentioned, 
I would add the following : 

H. C. HovEY. Celebrated A ??ierican Caverns. R. Clarke, Cincinnati, 1882. 

W. S. Blatchley. " Indiana Caves and their Fauna," in Geological Sur- 
vey of hidiana. Twenty-first A^inuai. Report, pp. 121-212, 1896. 



CHAPTER V 
STREAM DEPOSITS 

THE ability of a stream to carry detritus in suspension, 
as we have seen, varies as the sixth power of its 
velocity. The velocity depends mainly on the steepness* of 
the stream channel and on the volume of water. It is a 
familiar fact that streams vary in velocity throughout their 
courses. In general, mainly as a result of development, 
their channels are steepest and their waters swiftest near 
their sources, and become less steep and more sluggish as 
they near the sea; but throughout their length they are 
commonly broken into alternate swift and quiet reaches 
or sections. Evidently a stream may be able to transport 
all of the debris brought to it in certain portions of its 
course, and to corrade its channel, while in other portions 
it may be overloaded and consequently forced to drop 
a portion or even all of the material previously held in 
suspension. Evidently, then, streams both transport and 
deposit material as a part of their normal work. It is to the 
manner in which streams lay down their burdens and to the 
character and histories of the deposits thus formed that 
attention is now invited. 

Many streams, and especially large rivers, rise in mount- 

7 

97 



98 RIVERS OF NORTH AMERICA 

ains and flow across broad plains to the sea. It is con- 
venient to divide such streams into three sections in 
accordance with their velocities and with reference also to 
the topography of their borders. Although these divisions 
are really determined by the stage of development each 
stream has reached in various portions of its course, they 
are somewhat definite for such periods of time as man may 
have been acquainted with them. The three divisions re- 
ferred to are the mountain tract, where the w^aters usually 
flow impetuously in narrow, trench-like depressions; the 
valley tract, where a stream widens and is bordered by nar- 
row flood-plains ; and the plains tract, where the grade is 
still more gentle, and the stream meanders in broad curves 
through alluvial lands of its own making. 

In the mountains the streams, as a rule, are swift, and 
able to bear along not only fine debris in suspension, but 
to roll great boulders down their channels, especially during 
high-water stages. In the valleys through which the streams 
leave the mountains, the current slackens, and the coarser 
material brought from above is dropped. In the plains tract 
velocity is again diminished, and again a selection is made, 
the coarser portions of the burdens brought from above 
being deposited, and only the finest silt and sand ordinarily 
carried forward. 

The dropping of material in the valley and plains tracts 
of a stream leads to the filling in, or aggrading, of its 
channel in those parts. This means an increase in the gen- 
eral gradient of the stream channel below the mountain 
tract and consequently a swifter flow and an increase in the 
transporting power of the current. Coarse material is then 



Plate V. 




Statute Mi le a 



& <» a 2 X o 

Delta of the Mississippi ; by U. S. Coast Survey. 
Soundings on dotted areas in feet, all others in fathoms. 



STREAM DEPOSITS 99 

carried farther than before the stream bed was raised, and a 
new adjustment is made throughout the valley and plains 
tract. This process of aggrading will continue as long as 
the material brought by the swift head-waters of a stream is 
in excess of the transporting power of the current lower 
down. As will be shown later, a stream throughout much 
of its life deepens its channel in its mountain tract and de- 
posits all but the finest of the material thus removed in the 
lower and less swift portion of its course. 

The action of a stream in corrading its channel in one 
portion of its course and aggrading it in another portion, is 
carried on at the same time, and is a highly complex pro- 
cess. This complexity is again increased especially by 
variations in the volumes of the streams. During high- 
water stages a stream moves more material and carries it 
farther than during low-water stages. The result of these 
varying conditions may be seen in any stream that rises in 
high lands, and flows through a valley and across a plain. In 
the hill or mountain tract the stream channel is narrow with 
steep sides, and its bed comparatively free of fine debris, 
except in crevices and holes, although perhaps clogged with 
great stones too large for the water to move except during 
times of unusual flood. In the valley tract, where, for 
reasons to be considered later, the gorge through which the 
stream flows becomes wider, the stream bed is occupied by 
coarse gravel and small boulders, and much fine material 
may be found in the more sluggish reaches. Where the 
stream emerges into a plain, its bed is lined with fine sand 
and silt, except in the swifter reaches, where gravel occurs. 

This general decrease in the size of the debris dropped 



lOO J^ I VERS OF NORTH AMERICA 

by streams between their sources and their mouths would 
be much more regular than one ordinarily finds it, if it were 
not for the fact that velocity increases with volume, other 
conditions remaining the same, so that coarser material is 
carried during floods to localities where only fine material is 
normally dropped. As a rule, streams increase in volume 
from their sources to their mouths, but the increase in 
velocity due to this cause is, in most cases, more than 
counterbalanced by loss of grade. 

The debris that starts from the mountains on its journey 
down a stream channel to the sea makes many halts, and is 
gradually reduced in size both by mechanical wear and by 
solution. The periods of rest are due largely to variations 
in the volume of the stream, and also to its general decrease 
in grade from source to mouth. The larger stones are de- 
layed until finer debris washed against them and the solvent 
power of the water reduce their size so that the stream is 
competent to move them. The material laid aside is con- 
stantly undergoing chemical changes which decrease the 
size of the fragments by solution and tend to make them 
friable, so that they are more easily broken or worn when 
next exposed to mechanical agencies. Even during the 
more continuous portions of the journey of stream-carried 
debris, its onward movement is irregular, since particles 
carried upward by ascending currents are pulled down by 
gravity, and reaching the bottom may rest for a time until 
disturbed and again carried upward so as to feel the influ- 
ence of the general onward flow. For these and other 
reasons, the journeys of the debris transported by rivers are 
varied and usually long delayed. The removal of a given 



STREAM DEPOSITS lOI 

rock fragment from the mountains to the sea may require 
tens of thousands and even milHons of years. The drop- 
ping of debris by streams, or its lying aside until conditions 
for transportation are more favourable, leads to the origin 
of several varieties of deposits. 

ALLUVIAL CONES 

The influence of a decrease of slope in a stream's bed is 
revealed by deposits of debris in nearly every stream, but 
finds its greatest expression when sudden and excessive 
changes from high to low grade occur. When a stream de- 
scends a precipitous gorge in a mountain-side and emerges 
into a valley, there is an abrupt loss of power, and the 
greater part or perhaps all of the load that the swift waters 
in the mountain tract swept along, is deposited. A conical 
pile of debris is thus formed, the base of which is in the 
valley and the apex at the mouth of the high-grade gorge 
through which it was swept out of the mountains. (See 
Plate VI.) In America such piles of debris are termed allu- 
vial cones. In India * and in Europe they are generally 
known as alluvial fans, in reference to their fan-like forms 
when seen from above. As suggested by Gilbert, there is 
an advantage to be gained by retaining both of these terms, 
employing cone when the angle of slope is high, and fan 
when it is low. 

The best example of alluvial cones occurs in arid regions 
where precipitous mountains border desert valleys. They 

' An excellent account of alluvial fans in India, by Frederick Drew, may be 
found in the Quarterly Journal of the Geological Society of London^ vol. xxix., 
pp. 441-471, 1873. 



I02 RIVERS OF NORTH AMERICA 

are a common and characteristic feature in the scenery of 
the arid portions of the United States and Mexico, and are 
especially well displayed in Utah, Nevada, and Southern 
California. In that region the intermittent streams from 
the mountains are swollen during the infrequent storms and 
sweep along large quantities of debris. When the waters 
reach a valley and the grade of the stream abruptly de- 
creases, they lose velocity and much of their loads is 
dropped. In many instances, the waters on leaving the 
mountains where their channels are in solid rock and enter- 
ing alluvial-filled valleys are absorbed and percolate away. 
When this occurs, all of the debris brought from the mount- 
ain is deposited. Piles of debris are thus formed which 
are of all sizes up to one or two thousand feet or more in 
height, and with bases frequently three to five miles in 
radius. The slope of the surfaces of these piles varies with 
their size, the grade of the stream that deposited them, the 
nature of the material of which they are composed, and 
possibly other conditions. Their profiles along lines radiat- 
ing from their apexes are gently convex to the sky. The 
material of which they are composed varies in size from fine 
silt to boulders six or eight feet or more in diameter and 
weighing hundreds of tons, but in general they are com- 
posed of gravel and sub-angular stones and fine yellowish 
silt-like material. As the piles increase in size their bases 
expand and their apexes are extended farther and farther 
up the feeding gorges. 

In the arid portions of North America referred to, alluvial 
cones have been formed in such abundance and of such 
great size at the mouths of canyons on the borders of desert 



Plate VI. 



,.jillBk 




I 

I 



Fig. a. Sketch of Alluvial Cones. 




Fig. B. Indian Creek, near Taylorsville, California. 

The creek meanders through a flood-plain ; cut bank on the left in the foreground, and a 
sloping deposit of gravel on the opposite side of the stream ; these conditions are 
reversed at the second bend. 



STREAM DEPOSITS I03 

ranges, that they unite laterally in the valleys so as to form 
a fringe about the bases of the mountains. The contrast 
between thes& smooth, sloping pediments and the angular 
crags and peaks rising above them is frequently very marked, 
and adds an interesting feature to the peculiar scenery of 
the regions where they are best developed. The bases 
of the united alluvial cones are lobed, the greatest expan- 
sions being opposite the mouths of the larger canyons, thus 
giving to the bases of the desert ranges when seen from 
above a scalloped outline. Where the curving margins of 
the alluvial cones are drawn in toward the mountains sur- 
rounded by them, they meet projecting spurs and ridges 
from which there is but little drainage, and talus slopes fre- 
quently occur. The length and size of the gorges in the 
mountains may thus be judged by the extent and height of 
the alluvial deposits at their mouths. 

The streams reaching the alluvial cones, when not at once 
absorbed, build up their sides and channels, and thus be- 
come unstable. The waters break through the ridges formed 
along their margins, and branch or bifurcate in various 
directions, and are thus led from time to time over all por- 
tions of the surfaces of the cones, and distribute their bur- 
dens evenly upon them. This tendency of the supplying 
streams to bifurcate and send off distributaries is of the 
same nature, as will be noted later, as occurs on deltas. In 
fact, the upper portion or cap of a delta built by a high- 
grade stream, is an alluvial cone, having all of the charac- 
teristic features of the example under discussion. The 
abandoned courses of distributaries on the surfaces of 
alluvial cones are frequently marked by parallel ridges with 



I04 RIVERi OF NORTH AMERICA 

a stream bed between. The branching of the streams is 
also frequently well mapped in this manner during the long 
intervals between storms, when no water reaches the alluvial 
deposits. Sometimes huge boulders occur on the surfaces 
of the alluvial cones of the Far West, two or three miles 
from the mouths of the gorges from which they came, and 
remain as records of the violence of the streams during 
storms. 

Normally the surface of the alluvial cones, in the region 
just referred to, is bare of all vegetation except desert 
shrubs and grasses, while at their heads and in the mouths 
of the feeding gorges there are frequently clumps of willows, 
alders, and other less familiar shrubs. Occasionally the 
slight perennial drainage from the mountains, which is con- 
cealed by the debris in the bottoms of the gorges, comes to 
the surface at the head of an alluvial cone, and forms a 
spring towards which the cattle and game trails in the valleys 
converge. 

During the rainy season each year the gorges in the 
mountains are occupied by streams which bring down debris 
and make additions to the alluvial deposits, but in summer 
all contributions cease, except during occasional heavy rain- 
falls. The growth of the alluvial cones is thus markedly 
spasmodic. When the heavy rains, usually termed cloud- 
bursts, occur, torrents rush down the mountain gorges and 
carry with them vast quantities of debris which had been 
slowly accumulating in their channels, possibly for scores of 
years, and marked additions are made to the alluvial cones 
in the valley below. Between these occasional catastro- 
phes, disintegration of the rocks goes on, and the gorges 



STREAM DEPOSITS I05 

again become charged with rock fragments. During the in- 
tervals between floods, the winter streams frequently remove 
some of the material previously deposited, and a notch is 
cut in the apex of the alluvial cone. Many alluvial cones in 
Utah, Nevada, and adjacent regions, are thus notched at 
their summits, so that in ascending them in order to gain 
the gorge above, one passes through a trench in alluvium, 
possibly a hundred feet or more in depth. The sides of 
these gorges reveal sections of the alluvial deposit, and show 
that the material is rudely stratified. The strata are not 
horizontal, but inclined in conformity with the surface 
slopes of the cones, and show cross-bedding and many other 
irregularities. In general, the material composing an allu- 
vial cone is coarse near the entrance to the gorge to which 
it leads, and fine on its outskirts in the valley, but even on 
the lower borders coarse gravel and stones may occur. 

Within the gorges cut in the summit portions of these 
conical piles there are frequently terraces, which record 
various stages in the down-cutting performed by the 
streams. The reason why a small stream fed by winter 
rain and melting snow is able to excavate a channel in the 
apex of an alluvial cone that has been built up during 
occasional heavy storms, seems to be because during the 
normal winter flow the streams are less heavily charged with 
debris than at the time of the occasional floods, and can 
expend a portion of their energy in corrading. The light 
loads of the normal winter streams can, in some instances, 
be accounted for by the fact that the canyons have been 
cleared of available debris by a previous great storm. 
Another condition that may favour the cutting of the chan- 



I06 RIVERS OF NORTH AMERICA 

nel referred to, is that during times of slack drainage the 
waters are absorbed by the alluvium, and the silt brought 
by them is left in the interstices in the coarser debris, and 
the filling the interspaces causes the waters to be retained 
at the surface, thus allowing them to corrade. Yet another 
change in conditions which leads to the cutting of the chan- 
nels referred to, and one which occurs also, as will be con- 
sidered later, in the history of flood-plains, is a result of 
normal stream development. The removal of material from 
a mountain gorge when corrasion is in excess of weathering, 
causes a lowering of the gradient above the apex of the 
alluvial cone at its mouth, and necessitates a readjustment 
of grade in the lower course of the stream where deposition 
was previously in progress; this is accomplished by the 
cutting of a channel through the apex of the alluvial cone 
and the re-deposition of the material removed lower down 
on its surface. In such an instance a second cone is built 
on the surface of the first one formed, with its apex in the 
notch excavated in the summit of the earlier accumulation. 
Many alluvial cones are for this reason compound structures. 
Still greater complexity of the same character occurs in the 
flood-plain deposits of large rivers, as will be shown later. 

The fact that alluvial cones are more common and more 
conspicuous in arid than in humid regions depends on a 
variety of circumstances. One of the controlling conditions 
favouring their growth is an abrupt change in the grade of 
a stream bed. In humid regions, the grade of streams is 
more quickly adjusted than when the climate is arid and the 
streams intermittent. In humid regions, also, the streams 
are more constant, and, instead of terminating in desert 



STREAM DEPOSITS \Oj 

valleys, flow on and carry their loads to lakes or to lower 
regions instead of dropping them at one locality. Then, 
too, in humid regions, there are usually streams in the 
valleys bordering the mountains, to which the torrents are 
tributary. The valley streams tend to cut away and remove 
the alluvial deposit on their border, and thus prevent great 
accumulation of debris at the mouths of lateral gorges. 
Abundant rain-fall and vegetation also have an important 
influence on the deposits that may be formed. The rocks 
decay more rapidly, and sub-aerial deposits of debris are 
usually more quickly removed, in humid than in arid cli- 
mates, even when the mean annual temperature is the same. 
Vegetation not only masks the deposits of the nature here 
considered, but assists in various ways in their removal. 

The principal conditions favouring the origin and growth 
of alluvial cones are high-grade gorges leading to adjacent 
valleys in which there are no streams, and climatic condi- 
tions favourable to great fluctuations in the flow of the 
mountain torrents and also to rock disintegration rather 
than rock decay. 

As the rocks forming a mountain in an arid region are 
shattered and disintegrated by changes of temperature and 
other similar agencies, the loosened material is washed down 
the gorges and deposited in alluvial cones, in the manner 
just considered. As the alluvial cones about the base of a 
mountain gradually increase, their apexes progress farther 
and farther up the gorge, and should the favourable climatic 
conditions prevail sufficiently long, the alluvial deposits 
begun on opposite sides of a range may grow until they 
meet on the divide at the crest of the mountain. The dis- 



I08 RIVERS OF NORTH AMERICA 

integration of the still exposed peaks and ridges will con- 
tinue, and not only will the main valleys become choked 
with rock fragments, but each lateral gorge will also contain 
an extension of an alluvial cone. In time the exposed pin- 
nacles and ledges of rock will become covered with loosened 
fragments and the entire range assume subdued and flowing 
outlines. The core of still solid rock will then be covered 
and concealed by a sheet of disintegrated fragments, which 
will be thickest in the former gorges. There are mountains 
in the arid regions referred to above which have become 
buried in this manner in their own debris. 

The life histories of alluvial cones are long and remark- 
ably unvaried. The chief episodes in their lives are the 
sudden and severe storms which increase their rate of 
growth in a striking manner. They continue to increase in 
all of their dimensions as long as there is high land to fur- 
nish debris, unless climatic conditions change and the 
streams which supply them become perennial. If the 
climate remains arid, however, they gradually expand, per- 
haps with the engrafting of secondary or parasitic cones on 
their sides, until the mountain about which they began to 
form is buried, and then, owing to decay and the removal 
of material in solution, flatten out and become hills with 
gracefully flowing outlines, which in time slowly fade away. 

The reader will no doubt fancy that I have given too 
much space to the consideration of this special phase of 
stream deposition, and one which is of minor importance 
when all of the accumulations made by streams are con- 
sidered, but the study of alluvial cones assists in an import- 
ant way in understanding the nature of all other stream- 



Plate VII. 




Fig. a. Valley of Big Goose River, Wyoming. 
An old, alluvial-filled valley with a meandering stream, (Photograph by W. H. Jackson.) 




Fig. B. New River, Tennessee. 
A young valley excavated in a peneplain. (Photograph by M. R. Campbell.) 



STREAM DEPOSITS IO9 

made deposits. The upper portions of deltas, for example, 
are simply alluvial cones. Even the flood-plains of rivers 
are imperfectly developed, or, perhaps more properly, com- 
posite, alluvial cones with greatly extended sides and low 
surface slopes. The alluvial cones of arid regions and the 
alluvial caps of deltas are usually allowed to expand sym^ 
metrically, but flood-plains, as will be shown later, are con- 
fined by the sides of the valleys in which they are formed, 
and may be considered as long, narrow, longitudinal sections 
of deposits of a similar origin. 

TALUS SLOPES 

Alluvial cones frequently unite with or merge into the ac- 
cumulations of debris which form at the bases of cliffs and 
are supplied directly by rock fragments falling from the 
precipices above them. These talus slopes, or screes, as 
they are sometimes termed, are composed of angular frag- 
ments of rocks, although frequently rounded by weathering, 
of all sizes up to many cubic feet, and have surface slopes 
frequently as great as thirty or thirty-five degrees. Unless 
cut away at the base, as by a stream, the surface slopes re- 
present the angle of repose of the material of which the 
apron-like piles are composed. 

Talus slopes are not stream deposits, but are frequently 
associated with alluvial cones, and may be mistaken for 
them. They differ from alluvial cones, however, in their 
mode of origin, and in the fact that they are not composed 
of water-worn material, unless the cliffs above them contain 
conglomerate, and usually have much higher surface slopes. 



no RIVERS OF NORTH AMERICA 

The spaces between the large blocks are not usually filled, 
and the material of which they are composed is not stratified. 
These differences, and the fact that talus slopes occur at 
the base of precipices, while alluvial cones are built at 
the mouth of gorges, enable one to readily discriminate 
between them. 

FLOOD-PLAINS 

It is well known that streams, whether rills or rivers, 
seldom follow a straight course for any considerable dis- 
tance, unless held in a definite channel for a time by walls 
of solid rock. The path of a stream is a series of curves. 
Owing to the deflection of the thread of swiftest current, 
the banks on the concave or outer curves are eaten away, 
while those bordering the convex or inner curves are added 
to. As has been described in considering the meandering 
of streams, any inequality in their bottoms or banks may 
lead to a deflection of the current. If we imagine a stream 
flowing through a broad valley, even if of constant volume, 
this process of meandering from side to side in a tortuous 
course will go on. As the stream bed is shifted, deposits 
are laid down on the border of the convex curves, and new 
land formed. The first deposits made are of coarse material, 
usually well-rounded gravel or stones, next above this layer 
finer material is deposited, and last of all, fine silt. If the 
stream migrates from one side of its valley to the other, one 
bank is progressively cut away and the material thus re- 
moved in part re-deposited lower down on the other side, 
and at the same time assorted. In this manner a plain is 
formed, having an even surface of fine rich soil which 



STREAM DEPOSITS III 

changes towards its base into coarser and coarser material. 
When the meandering of the stream is reversed and it 
swings back again across the valley, the material it pre- 
viously deposited is again cut away and again carried lower 
down and re-deposited on the slack-water sides of lower 
curves, and is again assorted during the process. In this 
manner the material flooring a river valley is worked over 
and over many times. 

The manner in which streams cut away the land on the 
outer sides of stream-curves so as to produce steep banks, 
and deposit material on their inner or convex sides so as to 
form gentle slopes, is illustrated by the view of Indian 
Creek, California, presented on Plate VI. Other character- 
istic examples of meandering streams are shown on Plate III. 

In the case of a stream without marked variations in vol- 
ume, flood-plain building goes on slowly, or rather, perhaps, 
an approximate equilibrium is first reached and then the 
change is slow, but is probably always in progress, although 
varying in different instances according as the stream is 
clear or more or less charged with sediment. In nature the 
process just outlined is almost always assisted by variation 
in the volume of a stream. During periods of decreased 
rain-fall, as, for example, in the summer season in most 
regions, the streams are low and confined to their immediate 
channels; when storms come, however, or the snow melts, 
the streams are swollen and more than fill their summer 
channels. It is during floods that the greater amount of 
corrasion on the concave curves and of deposition on the 
convex curves takes place. 

There are certain limitations to the extent a ?5tream may 



112 RIVERS OF NORTH AMERICA 

meander to the right and left of its general course, inherent 
in the process by which curves are formed, which deter- 
mine the width of the belt of country it works over. In 
studying these limitations, however, a distinction should be 
borne in mind between the comparatively small curves 
originating in the manner just described, and the much 
greater sweeps, made up of many small curves, which I 
have termed migrations, as, for example, the great swings 
of the Mississippi from side to side of its broad alluvial 
valley below the mouth of the Ohio. The checks men- 
tioned below on the tendency of a stream to meander in 
small, sharp curves do not seem to be operative, or at least 
do not act in the same manner, in the case of the broad 
migrations. 

When a stream forms an oxbow curve, it is plain that its 
course is lengthened, and that aggrading must take place in 
order to allow the waters to carry their burden over the ex- 
tended course. The bed of the stream at the beginning of 
a curve will thus be raised higher and higher, as the curve 
increases in length. During times of flood when the stream 
overspreads its banks, the waters cross the narrow neck of 
the oxbow curve and begin to excavate a channel. Once a 
start is made the entire river soon takes the shorter and 
steeper course, and the curve is cut off. The entrance 
and exit of the abandoned curve are soon silted up, on ac- 
count of the deposition of debris in the slack water in the 
embayments on the side of the straightened stream, and 
an *' oxbow lake " is the result. 

As cited by C. R. Keyes,' the difference in elevation on 

^Missouri Geological Survey, vol. x., p. 97, 1896. 



STREAM DEPOSITS II3 

the two sides of the neck of an oxbow curve is frequently 
sufficient for a small ravine to be cut backward from the 
lower side so as to make a slight rent in the narrow strip of 
land, thus directing and facilitating the work of the over- 
flowing waters when a flood occurs. 

There is thought to be a definite limit established in this 
manner on the extent to which a stream may meander, but 
on account of the numerous varying conditions entering 
into the problem, — such as the volume of a stream, its 
velocity, its fluctuations, the influence of vegetation, the 
nature of its banks, and in many regions the influence of 
ice, — it has not been found possible to determine how 
widely any given stream may meander. 

The broad migrations referred to above depend on con- 
ditions still more complex than those limiting a stream's 
meanderings, and as they have not been carefully studied, 
it is perhaps best not to attempt to discuss them at this 
time. 

During floods, also, a stream frequently inundates the 
plains formed during previous meanderings. The region 
thus submerged is for this reason termed d. flood-plain. The 
waters on spreading from the main channel lose velocity, 
and the silt they hold in suspension is, to a great extent, 
precipitated. A layer of fine material is thus spread over 
the plain and raises it by the addition of a layer of rich soil. 

The debris brought from the upper portion of a stream 
course, where, on account of the high grade, corrasion is in 
progress, is thus dropped in the valley and plain tracts, and 
not only raises the bed of the stream but is built into flood- 
plains. 

8 



114 RIVERS OF NORTH AMERICA 

Flood-plains of the character just described border prac- 
tically all the streams of North America, below the region 
of active corrasion, and furnish some of the richest and 
most easily cultivated lands the continent possesses. The 
flood-plains of the Mississippi, for example, are from ten to 
fifty or sixty miles broad, and in the neighbourhood of fifty 
thousand square miles in area. In the settlement of a new- 
country the flood-plains are usually the first areas occupied, 
not only on account of their fertility, but because of their 
ready access. The main obstacle met with by early settlers 
is usually the heavy growth of timber that has to be cleared 
away in order to admit of the cultivation of the soil. 
Another difficulty arises from the fact that the flood-plains 
are subject to inundation at times of high water, as is well 
known in the case of the lower Mississippi. After the floods 
have subsided, the moist lowlands are in many instances 
unhealthy and malarial fevers are apt to be prevalent. 

It is of interest to note that flood-plains begin to form 
where a stream loses grade and is no longer able to trans- 
port the debris brought down its higher-grade upper course. 
Usually they first appear as one descends a river, at the 
lower end of its mountain tract, and extend from that 
locality to the sea. Their similarity to alluvial cones is 
thus made apparent. An alluvial cone is in fact a flood- 
plain, formed under special conditions. The surface slopes 
of alluvial cones are high in comparison with what may be 
termed normal flood-plains, because the gorges of the short 
streams supplying them are of higher grade than the larger 
streams which build long flood-plains. The purpose, as we 
may say, of the alluvial cone is to grade up the lower course 



STREAM DEPOSITS Ilg 

of a stream to conform with its slope in its upper portion, 
and thus furnish an even grade down which the stream may 
carry its burdens. The same is true of flood-plains; the 
river has more material delivered to it at the head of its 
valley tract than it can continue to carry with the current 
necessitated by the lower slope farther downstream, and at 
once begins to raise its grade. The aggrading progresses 
all the way from the locality where the stream begins to be 
overloaded, throughout its lower course to the sea, unless 
rapids break its course. When rapids occur, each separate 
portion of the stream behaves like an independent river, 
and each section deepens its channel or fills it in, as the 
case may be. 

In an alluvial cone the greatest depth of the deposit is 
near its head, and probably at the locality where deposition 
first began. In a flood-plain, in general, the greatest depth 
of the deposit is near the lower end of the mountain tract 
and decreases all the way to the sea. The locality of 
greatest depth of filling, however, will migrate up stream as 
the flood-plain increases in length. Unless the stream is 
building a delta and thus lengthening its course, the flood- 
plain at its mouth cannot be raised. Hence deposition on 
a flood-plain at sea-level is practically nil. 

If the changes experienced by a stream from youth to 
old age are followed, it will be found that its valley, plains, 
and mountain tracts are mutually extended up stream. The 
plains tract especially increases in length, while the mountain 
tract becomes shorter and shorter unless it is so situated 
that it can extend its branches headward. With the 
lengthening of the portion of the stream characterised by 



Il6 RIVERS OF NORTH AMERICA 

low gradients — that is, of the plains and valley tracts, — there 
is an up-stream migration of the locality where flood-plains 
begin. These normal phases of stream development, how- 
ever, w^ill be more readily understood after the adjustment 
of streams to the structure of the rocks corraded by them 
has been considered. The fact that a stream under certain 
conditions, already noted, cuts a channel through the upper 
portion of an alluvial cone and forms terraces in the gorge 
thus produced, will assist us later in understanding the man- 
ner in which streams may cut channels through their flood- 
plains, and form terraces on the side of such channels. 

NATURAL LEVEES 

On visiting a stream that is subject to floods, and carries 
a heavy load of silt especially during high-water stages, one 
will usually find that its immediate banks are higher than 
the general level of the surface of the adjacent flood-plain, 
and form ridges. In the case of creeks and small rivers, 
the ridges may be a hundred feet across, and their crests 
five to six feet or more above the surface of the flood-plain 
or back country, as it is frequently termed. In many in- 
stances, too, the back country between the river and the 
bluffs bordering the valley, is imperfectly drained and in a 
swampy condition. The region referred to is frequently 
densely covered with alders and other swamp-loving plants, 
and may support great cottonwoods and other forest 
growths. Through the forests one frequently finds open, 
park-like passages, clear of vegetation, which mark second- 
ary channels, occupied by branches of the main stream 



STREAM DEPOSITS 11/ 

during high-water stages. The embankments referred to 
on each border of a stream, where it flows through a flood- 
plain, are built up by the river. As the waters rise during 
floods, and spread beyond the border of the low-water 
channel, the current is checked, and they drop all but the 
very finest of the material held in suspension. It thus hap- 
pens that the thickest deposits are made on the immediate 
border of the channel, and ridges which follow all the mean- 
derings of the stream are built up. These embankments 
resemble the levees built by man in attempting to confine a 
stream to its immediate channel, and are hence known as 
natural levees. 

When the waters of a rising river pass the natural levees, 
they are still charged with silt, much of which is precipitated 
on the inundated country and adds to the layer of fine ma- 
terial found on the surface of all low-grade flood-plains. It 
is to the addition of this thin layer of exceedingly fine 
mud, mingled in many instances with organic matter, derived 
from the working away of soil farther upstream, that the 
wonderful fertility of flood-plains is largely due. The decay 
of the rank vegetation on many flood-plains, especially 
during the time previous to the clearing of the land for 
cultivation, also adds to the fertility of ** river bottoms," as 
the flood-plains are frequently termed. 

While natural levees are a protection to the back country 
during moderate floods, serving as they do to retain the 
waters in a definite channel, yet, as has been found, espe- 
cially in the case of the Mississippi, they are also a source of 
danger. During unusually high floods, the rivers break the 
embankment confining them, and branching streams are 



Il8 RIVERS OF NORTH AMERICA 

formed, which carry destruction in their paths and sweep 
away the fields and whatever else obstructs their flow. 

The typical cross-profile of a river valley in which levees 
occur, as has been shown by Hicks, ^ does not present a 
straight line from the stream's channel to the bordering 
bluffs, but a double curve. If no terraces are present on 
the sides of the valley, each half presents a weather-curve 
at the top of the bluff, and two other curves of opposite 
character, one concave due to erosion, and the other convex 
due to deposition. At the intersection of the water-curves 
there is usually a swamp. These features, somewhat ex- 
aggerated in the vertical scale, are shown in the following 
diagram. 




Fig. 4. Cross-Profile of a Valley Occupied by a Constructive River. After 

L. E. Hicks.) 

a b. Weather-Curve at Crest of Bluffs; b c. Water-Curve of Corrasion ; c d. Swamp; de. 
Water-Curve of Deposition ; j. Stream Channel. 

The water which covers a flood-plain between the levees 
and the uplands bordering a valley during flood, finds its 
way back to the river again at some locality lower down- 
stream, usually by entering a tributary. In the lower Mis- 
sissippi, much of the water that escapes in this manner 
from the main channel is carried to the Gulf of Mexico by 
distributaries from the main stream. 

Where natural levees are built, the bed of a stream is 

' L. E. Hicks, " Some Elements of Land wSculpture," in Bulletin of the Geo- 
logical Society of America, vol. iv., p. 143, 1893. 



I20 RIVERS OF NORTH AMERICA 

usually raised by deposits made in its bottom. In this way 
it happens that a stream sometimes flows in a trench on the 
top of a ridge more elevated than the back country, and is 
thus in an unstable position, and liable to shift its channel 
when breaks in the levees occur. This is one method by 
which streams are led over all portions of the valleys they 
occupy. 

Illustrations of the manner in which a river subject to 
floods breaks through its levees from time to time and in- 
undates its flood-plain are furnished nearly every year by 
the Mississippi, especially below the mouth of the Ohio. In 
the case of this river, however, the extent of the inundation 
is increased in many instances by the attempts that have 
been made to confine it to its main channel by adding to 
the height of the natural levees. Under the artificial con- 
ditions imposed on the river, when breaks, or crevasses, as 
they are termed, in the levees do occur, plantations are 
ruined, buildings swept away, and in some instances the 
moUusks, fishes, and other animals in the bays and sounds 
bordering the deltas are destroyed by the vast quantities of 
fresh water, charged with mud, poured into them. 

The extent of the inundation in the lower Mississippi, in 
the spring of 1890, is shown on the accompanying map. 
The shaded portion of the map indicates the regions that 
were covered by the overflowing waters, while the unshaded 
portion reveals the distribution of the land too high to be 
reached by the flood. It is instructive to note that the un- 
submerged land lies close along the borders of the streams, 
and indicates the position of their embankments. At the 
time of the inundations referred to, the water broke through 



STREAM DEPOSITS 121 

the left embankment of the river at what is known as the 
Nita crevasse, about twenty miles above New Orleans, and 
formed a current of fifteen miles an hour, which carried de- 
struction in its path. The escaping water, joining that 
from another break known as the Martinez crevasse, ap- 
proximately twelve miles in a straight line higher up the 
stream, flowed eastward and caused Lake Maurepas and 
Lake Pontchartrain (six hundred square miles in area) to 
overspread their banks. The water flowed over the country 
eastward to Lake Borgne and entered Mobile Bay, with 
such volume as to cause a current eastward and destroy for 
a time the important oyster and fish industries of that arm 
of the sea.^ 

The study of flood-plains and the manner in which 
streams divide near their mouths when aggrading, or delta- 
building, is in progress, illustrates a natural method by 
which inundations of the country bordering the lower course 
of an alluvial river are lessened or prevented. When a dis- 
tributary leaves a trunk stream, the volume of water below 
the place of escape is lessened, and the tendency to break 
across the levees decreased. The practice of building 
artificial levees, so frequently resorted to in the vain at- 
tempt to control large rivers subject to high-water stages, 
checks this natural tendency. If, for example, instead of 
attempting to confine the Mississippi to a single channel in 
its lower course, the natural distributaries could be cleared 

' L. C. Johnson, " The Nita Crevasse," in Bulletin of the Geological Society 
of America^ vol. ii., pp. 20-25, i8gi. 

The areas overflowed by the Mississippi in 1897 are shown on an excellent 
map forming Plate II. of Park Morrill's report on the " Floods of the Mississippi 
River." Published by the Weather Bureau, Washington, 1897. 



122 RIVERS OF NORTH AMERICA 

of driftwood or other obstructions and enlarged, the excess 
of water during floods might be drawn off, thus lessening 
the danger to the levees farther seaward. 

The influence of natural levees on the geography of a 
valley where aggrading is in progress, receives additional 
illustration from the manner in which tributaries of the main 
stream are deflected from what would otherwise be their 
natural courses. The building up of the borders of the 
main stream necessitates important changes in its tribu- 
taries. If the main stream increases the height of its levees 
at a greater rate than its tributaries can aggrade their chan- 
nels, evidently their waters must either rise so as to discharge 
across the dam thus formed, or be turned aside and flow more 
or less nearly parallel with the main stream until conditions 
favour a junction with it. The most common cause for such 
a union lies in the fact that each stream is meandering, and 
sooner or later one or the other will cut into the embank- 
ment built by its neighbour. The origin of lakes along Red 
River, Louisiana, owing to the raising of the levees on its 
margins faster than its tributary streams are able to aggrade 
their channels, has been described by Davis/ An examina- 
tion of a good map " of the lower Mississippi will show that 
its tributaries below the mouth of the Ohio frequently ex- 
hibit an abrupt change in direction after entering the broad 
valley of the main river, and flow parallel with it for many 
miles before being able to effect a junction. The various 

* W. M. Davis, Science, vol. x., pp. 142, 143, 1887. 

^ An excellent map of the lower Mississippi valley in eight sheets, scale five 
miles to an inch, has been published by the U. S. Mississippi River Commis- 
sion, price forty cents. This can be obtained by applying to the Secretary, 
Mississippi River Commission, St. Louis, Mo. 



STREAM DEPOSITS 1 23 

branches of the Yazoo, for example, after entering the 
valley of the Mississippi, flow nearly parallel with that river 
for about two hundred miles, but join it at Vicksburg, 
where the main river curves eastward and washes the base 
of the bordering bluffs, leaving no room for a secondary 
and parallel stream. 

DELTAS ' 

When streams deliver their loads of detritus to bodies of 
still water, either lakes or the ocean, deposition takes place 
in much the same way as when alluvial cones are formed, 
but the structure and shape of the deposit are influenced in 
an important manner by the presence of the still water into 
which the stream discharges. These deposits, when seen 
from above, as represented on a map, have the shape of an 
open fan. The curved margin of the fan faces the open 
water, and the sharp apex, or handle, is situated at the 
mouth of the feeding stream or well up its valley. The 
deposits laid down in the Mediterranean by the Nile have 
the characteristic form referred to, except that the fan is not 
fully open ; that is, the outline of the delta is a triangle and 
resembles the Greek letter A, as was long since noted ; 
hence the generic name still in use. All deltas do not ex- 
hibit the characteristic form of the type example, however, 
but may be markedly semicircular on their outer margin, or 
variously lobed or indented, as is shown by the compound 

^ The account of deltas here presented is essentially the same as may be found 
in the author's book entitled Lakes of North America^ one of the series of 
reading-lessons to which the present volume belongs, but this repetition is 
thought desirable in order to make each book as nearly complete in itself as 
practicable. 



124 RIVERS OF NORTH AMERICA 

delta of the Mississippi. An inversion of the A form in the 
case of certain small deltas in Seneca Lake has been de- 
scribed by the writer.* 

The conditions governing the formation of deltas are 
mainly that a stream shall bring detritus to a body of water 
which is unaffected by strong currents. If currents exist in 
the receiving water-body where the stream enters, capable 
of bearing away the debris delivered to it, the plan of the 
delta will be modified, and if the currents are sufificiently 
strong, all of the material is carried away and deposited 
over the bottom or built into bars and embankments of 
various forms along the margin of the lake or ocean. It is 
sometimes stated that deltas are not formed in water bodies 
affected by a tide, but this is an indirect explanation. To 
be sure, deltas are seldom found along the shores of the 
ocean where the rise and fall of the tides are well marked, but 
this is for the reason that the tides usually cause currents 
which bear away the debris brought by streams as fast as 
delivered. Water bodies sufficiently quiet to favour the 
growth of deltas are not necessarily tideless. Currents in 
water bodies are also produced by the wind, and may in- 
fluence delta-building in a similar way to tidal currents. 
For these reasons, sheltered bays and estuaries, where the 
fluctuations of level due to winds and tides are not excess- 
ive, are among the most favourable places for the growth of 
deltas. 

In the study of deltas, it is convenient to divide them 
into two classes, namely, those made by high-grade and 
consequently rapid streams, and those made by low-grade 

^ I. C. Russell, Lakes of North America, pp. 48-51. Ginn & Co., 1S95. 



STREAM DEPOSITS 1 25 

and therefore comparatively sluggish streams. These dis- 
tinctions apply of course only to the lower coui*ses of the 
delta-making streams. Between the two types referred to, 
a complete gradation may be found. 

Deltas of Higli-Grade Streams, — Typical examples of 
deltas of this class occur in Utah about the borders of 
ancient Lake Bonneville/ and have had gorges cut through 
them since the lake surface was lowered. A radial section 
of a delta of this type, such as would be exposed in the 
walls of a trench cut through it from apex to outer margin, 
is shown in Fig. 6, page 126, which will assist the reader 
in understanding the leading characteristics of such deposits. 
A swift stream is able to bring to the still waters of a lake, 
or of the sea, a heterogeneous load of detritus. On enter- 
ing the receiving water-body, this detritus is more or less 
perfectly assorted. The coarser or heavier portions, namely, 
the boulders, gravel, and coarse sand, fall to the bottom, 
while the lighter and finer particles, that is, the fine sand 
and silt, are carried farther. The very fine silt is floated 
away from the mouth of the feeding stream, and slowly 
settling to the bottom forms an addition to the sheet of 
mud which is being laid down in nearly every water-body. 

The coarse material referred to, dropped at the mouth of 
the stream so as to form a delta, makes an addition to the 
land and the stream channel is lengthened. The outer 
border of this deposit is steep for the reason that the debris 
as it is dropped tends to form a pile much the same as when 
similar material is heaped up on the land, the supply being 

^ G. K. Gilbert, " Lake Bonneville," U, S. Geological Survey, Monographs ^ 
vol. i., pp. 65-70, 1890. 



126 



RIVERS OF NORTH AMERICA 



from the top. The angle of repose, that is, the surface 
slope of material deposited under water, is steeper than 
would be assumed by the same material accumulated in a 
similar manner on land, and some assorting takes place. 
Additional material brought by the stream is carried to the 
top of the steep submerged slope referred to, and rolls down 
it so as to form an addition to the inclined layers. As a delta 
grows in all directions from the mouth of the feeding 
stream in which the water has freedom to flow, and invades 
deeper and deeper portions of the receiving water-body, 
the accumulation of inclined layers becomes broader and 
broader at the same time that its outer slope or under- water 
escarpment increases in height. The angle which the 
escarpment makes with a horizontal plane varies with the 
size and shape of the material composing it, but in most 
instances is in the neighbourhood of thirty-five degrees. 




Fig. 6. Radial Section of a Delta Built by a High- Grade Stream. 

The inclined layers, as indicated in Fig. 6, terminate 
upward in a horizontal plane, which coincides with the sur- 
face of the receiving water-body. As is indicated in the 
diagram, a delta built by a high-grade stream has three 
well-defined portions; the middle member of the series 
being the debris laid down in inclined layers, in the man- 
ner just described. 

The fine material delivered by the feeding stream is car- 
ried beyond the outer margin of the system of inclined beds 



STREAM DEPOSITS \2J 

forming the medial member of the delta, and subsides to 
the bottom. This material is also assorted. Tlie coarser 
and heavier portions reach the bottom at the immediate 
base of the escarpment formed by the inclined layers, while 
the finer portions are carried farther before coming to rest. 
In this manner a conical deposit of fine material with a low 
surface slope is made about the base of the accumulation 
of inclined layers. As the delta grows, the medial member, 
that is, the system of inclined beds of coarse material, 
advances over the finer deposits accumulated about its 
base, and in many instances causes them to be pressed out- 
ward and variously disturbed. The layers of fine material 
are frequently folded or broken by the weight of the body 
of coarse debris as it advances upon them. 

The third or superior portion of the delta, or the delta 
cap, as it may be called, is an alluvial cone which is built 
on the plane formed by the truncated edges of the inclined 
layers in the medial portion. The apex of the cone is well 
up the feeding stream from the locality where the delta first 
began, and as the delta increases in size, migrates up stream 
at the same time that the alluvial cone increases in all of 
its dimensions. 

The feeding stream in flowing down the surface of the 
delta cap is deflected, or caused to divide, from time to 
time, on account of the deposits made on its bottom and 
sides, and sends off branches in the same manner as has 
been explained in the case of alluvial cones, and at one 
time or another flows over all portions of the surface of the 
delta and discharges at all points about its outer border. 
The delta cap is thus built up, and the periphery of its base 



128 RIVERS OF NORTH AMERICA 

exfended by the addition of material to its surface. As in 
the case of alluvial cones, the surface slope of the delta cap 
is controlled by the grade of the feeding stream, but be- 
comes less and less steep as the area of its base increases. 
The more or less definite layers of which it is composed are 
inclined and slope downward towards the receiving water- 
body. As each layer was added to its surface at a certain 
stage in its growth, the inclination of those first formed is 
somewhat greater than the slope of those deposited later. 
An inclination of three to five degrees with a horizontal 
plane is common in deltas of the class here considered. If 
we visit a growing delta in process of construction by a 
high-grade stream, it will be found that the stream is bring- 
ing both coarse and fine material to the apex of the delta 
cap. The coarser portions of this load are dropped on the 
exposed surface of the delta, together with much fine ma- 
terial which serves to fill the interspaces between the boul- 
ders and larger stones and tends to prevent loss of water 
by percolation. This building up or aggrading serves to 
adjust the grade of the feeding stream as its length increases 
owing to the outward extension of the delta. Both coarse 
and fine material is carried down the slope of the delta cap 
and delivered to the receiving water-body, where it is 
assorted, the larger and heavier debris falling at once and 
adding to the series of inclined layers previously deposited, 
while the finer material is carried beyond the periphery of 
the delta and distributed over the adjacent bottom or 
floated away, as already stated, and finally brought to rest 
with other fine sediment on the floor of the basin occupied 
by the still waters. 



STREAM DEPOSITS 1 29 

In the passage of the debris down the surface of the delta 
cap an assorting in reference to form may frequently be 
recognised. The flat and angular stones come to rest under 
conditions that allow rounded and well-worn stones of simi- 
lar size and density to be rolled along. The highly inclined 
layers in a delta are usually composed of well-rounded 
stones. 

Referring once more to the radial section of a delta given 
above, it will be understood that the upper portion or delta 
cap consists of gently sloping, cross-stratified, and irregular 
beds of both coarse and fine material, the larger stones being 
frequently angular; the medial member is composed of 
steeply inclined layers, usually of well-rounded stones, 
mainly gravel and sand ; the basal member is of fine sand 
and clay, deposited originally in nearly horizontal sheets 
which decreased in thickness in a direction opposite to the 
source of supply, but later were pressed into folds, or broken 
and displaced, owing to the heavy weight imposed upon them. 

It should be noted that the growth of a delta increases 
the length of the supplying stream, and in order that it may 
carry debris to its advancing terminus, the newly occupied 
territory must be built up or aggraded. This process of 
filling in causes the apex of the alluvial cap to migrate up 
stream. 

At an advanced stage in the building of a delta, the 
lengthening of the course of the feeding stream would de- 
crease its rate of flow, and at the same time the deepening 
of the stream's channel above the delta, owing to corrasion, 
would tend in the same direction. These changes would 
necessitate a readjustment of grade which might lead to the 



I30 RIVERS OF NORTH AMERICA 

cutting of a channel through the apex of the delta in the 
same manner that the apexes of alluvial cones are sometimes 
notched. 

Deltas of Low-Grade Streams. — Typical examples of 
the deltas of this class are furnished by the deposits now 
being laid down at the mouth of the Mississippi, Yukon, 
Mackenzie, Nile, Ganges, and many other streams that have 
a low grade for a long distance near their mouths and de- 
liver only fine sediment to the still waters into which they 
flow. In these deltas the three divisions so characteristic 
of the similar deposits of high-grade streams cannot be 
recognised. All of the material delivered by low-grade 
streams is fine, mostly of the nature of fine sand and silt, 
and no marked assorting takes place and no distinctions 
between gently and steeply inclined layers can be distin- 
guished. The tendency of low-grade and mud-charged 
streams is to make broad deposits with indefinite borders 
rather than thick, well-defined accumulations. 

The debris brought to still waters by the class of streams 
here considered, is dropped as in the case of high-grade 
streams, but, being fine, a larger proportion is carried to a 
distance from the shore. Much debris is dropped as the 
incoming stream meets still water, but the angle of repose 
for fine sand and silt is much less than for coarse gravel, and 
the outer slope of the deposit is not well defined. More- 
over, fine material is easily disturbed by currents in the re- 
ceiving water-body, which again is unfavourable for the 
formation of sharply defined delta margins. Low-grade, 
muddy streams shoal the still water at their mouths and 
gradually form new land, which, however, is but slightly 



STREAM DEPOSITS I 3 I 

raised above the surface. The length of the stream is thus 
increased, which necessitates a new adjustment 'of the 
grade for a long distance up its course. Owing to the 
gentle grade of the stream, the changes produced in this 
manner are not pronounced. As the length of the stream 
is extended, the natural levees are also prolonged. When 
floods occur, breaches are made in the levees, and the stream 
divides, and sends off distributaries which discharge inde- 
pendently. Each distributary builds a pair of embankments 
or levees, and also an independent delta. The results of 
this process of subdivision are well illustrated at the mouth 
of the Mississippi, as may be seen from the accompanying 
map (Plate V.). Each of the finger-like extensions of the 
delta is due to the prolongation of a pair of embankments 
into the Gulf, by each distributary, and the growth of a 
secondary delta at its mouth. The Mississippi is thus 
building a compound delta, composed of the secondary 
deltas formed at the mouths of the several distributaries of 
the main river. The fact that each distributary is forming 
a delta of its own is best illustrated perhaps at the mouth 
of what is known as Cubit's Gap, a break in the left levee, 
about four miles upstream from where the main '' passes " 
or distributaries diverge. 

The branches of the secondary deltas, with their levees, 
frequently join, and the low spaces between them become 
transformed into lakes. Delta lakes, like Pontchartrain to 
the north of New Orleans, are thus formed. Several lakes 
of this type are shown on the accompanying maps (Fig. 5 
and Plate V.), as well as a number of bays, which will evi- 
dently be shut off from the sea in time and become lakes. 



132 RIVERS OF NORTH AMERICA 

The surface of the delta of a low-grade stream is in reality 
an alluvial cone, but of such a low slope that the eye cannot 
usually distinguish it from a horizontal plane. The exposed 
portion of such a delta, with its many levees, corresponds, 
both in the manner of its formation and in the alluvial 
nature of its material, with the delta cap of a high-grade 
stream. As in the type of delta first considered, the exten- 
sion of the delta increases the length of the feeding stream, 
and necessitates a grading up of the extended portion of 
the river channel so as to furnish the requisite slope for the 
transportation of debris to the advancing extremity. The 
surface of the delta is also raised, and its apex migrates up 
stream. 

The apex of a low-grade delta is usually indefinite, and 
its position difficult to determine. For this reason in part, 
such a delta is usually considered as beginning when the 
first distributary having an independent course to the re- 
ceiving water-body is given off. Under this definition, the 
head of the delta of the Mississippi is near the mouth of 
Red River, or about two hundred miles in a direct line from 
its extreme southern end. In reality, the apex of the delta 
cap is much farther upstream. The area of the Mississippi 
delta, as determined by Humphreys and Abbot, is one thou- 
sand two hundred and thirty square miles. The depth of 
the deposit, as shown by recent borings at New Orleans, is 
over one thousand feet,^ but, as will be seen later, this great 
depth is due to subsidence and the superposing of one delta 
on another. 

' E. L. Corthell, *' The Delta of the Mississippi River," in The N^ational Geo- 
graphic Magazine^ vol. viii., p. 351, 1897. 



STREAM DEPOSITS 1 33 

A delta, comparable in many ways with that of the Mis- 
sissippi, has been formed by the Yukon in Bering Sea; the 
distance in this instance from where the first distributary is 
given off to the periphery of the delta, is about one hundred 
miles. The outer or seaward margin of the deposit measures 
about seventy miles. The land between the several distribu- 
taries is swampy, and natural levees are less conspicuous than 
in the case of the Mississippi. The surface of the delta, a 
glimpse of which is given in Fig. A, Plate II., is treeless, 
but covered with a luxuriant growth of grasses, rushes, 
and low flowering annuals, and is a luxuriant garden of 
flowers in early summer, but at a depth of a few feet the 
soil, as has previously been mentioned, is always frozen. 
The delta in fact is a part of the extensive frozen marshes, 
or tundra, which border the shores of Bering Sea and the 
Arctic Ocean. No survey of this delta has been made, but 
a most instructive example of the nature of the deposit 
formed by a heavily loaded river, subjected to great 
inundation, there invites the student of geography. 

A great delta is also being extended into the Arctic Ocean 
by the Mackenzie. The Colorado is filling in the Gulf of 
California, and many smaller streams are making delta de- 
posits in the lakes of North America. In each of these in- 
stances some phase of delta-building is apt to be more 
prominent than others, but in almost any example that can 
be chosen the general laws governing the deposition of 
material brought by loaded streams when their currents are 
checked by still water may be observed. 

An abnormal delta, and one of interest on account of its 
novelty, is being made in Lake St. Clair by the St. Clair 



134 RIVERS OF NORTH AMERICA 

River. The river referred to is the outlet of -Lake Huron, 
and as lakes act on settling basins, it would be expected that 
the stream draining it would be clear and therefore incapable 
of forming a delta. The shore currents in Lake Huron, 
however, bring debris to the place of outlet, and deliver it 
to the draining stream. This material is carried to Lake St. 
Clair and there deposited in a broad delta, with several dis- 
tributaries, similar in some of its features to the delta of 
the Mississippi.* 

Many of the large streams of North America are not 
making conspicuous deltas for various reasons. The St. 
Lawrence, for example, is a clear stream, and therefore has 
but little material to deposit when its current is checked. 
Like a large number of rivers flowing to the Atlantic, 
the St. Lawrence, as already stated, has been greatly 
affected at a comparatively recent date by a subsidence of 
the land, which has allowed the ocean to extend far up its 
valley, and to submerge the deltas it may have previously 
formed. Other streams on the Atlantic border of the 
continent, like the Hudson, Delaware, Susquehanna, Poto- 
mac, and James, also enter estuaries, but are not clear 
streams. The absence of conspicuous deltas about their 
mouths is due to currents in the receiving water-bodies, 
and to the recency of the drowning of their lower 
courses. 

Effects of Changes in the Elevation of the Land on the 
Growth of Deltas, — A rise or a subsidence of the land along 
the ocean's shore where deltas are being formed, has the 
same effect on their growth as the lowering or raising of 

' I. C. Russell, Lakes of N'orth America, p. 40. Ginn & Co., Boston, 1895. 



STREAM DEPOSITS 1 35 

the surface of a lake where similar additions to the land are 
being made. 

If the surface of a lake rises after the building of a delta 
on its borders is well under way, the deposit may be par- 
tially or wholly submerged and a new structure of similar 
character formed above it. If the waters of a lake subside 
after a tributary stream has built a delta, its outer or sub- 
lacustral escarpment will be more or less completely ex- 
posed. The waters of the stream will descend this slope 
with accelerated velocity and corrade. Of the two or more 
subdivisions into which the stream may be divided on the 
delta cap, previous to the change just assumed, some one 
branch, either by being shorter than the others, or by hav- 
ing a greater volume, will deepen its channel more rapidly 
than its competitor and draw off its water, so that but one 
trench will be cut in the emerged delta. If the surface 
of the lake is depressed below the level of the base of the 
delta it will be completely cut through so as to expose a 
section of the deposit and a portion of the material on 
which it rests. The material removed during the cutting 
of the gorge, together with fresh debris brought by the 
feeding stream from above the old delta, will be again de- 
posited and a new delta built at a lower level. The apex 
of the second delta will be in the gorge cut through 
its predecessor or below the base of the old delta accord- 
ing to the extent principally to which the lake surface is 
lowered. 

Variations in the stages of delta-building, owing to 
changes in the elevation of the land, or of the level of a 
lake, as well as modifications of the process which would 



136 RIVERS OF NORTH AMERICA 

follow a change in grade, or in the volume or load of the 
feeding stream, may be readily determined. 

VARIATIONS IN NORMAL STREAM DEPOSITIONS 

In the preceding portion of this chapter we have con- 
sidered, for the most part, the various processes by which 
streams lay aside their burdens when not influenced by dis- 
turbing conditions. Let us glance at what may be termed 
the accidents that sometimes modify or interrupt the normal 
processes of stream deposition. 

Influence of Elevation and Depression of the Land on 
Stream Deposition, — The velocity of a stream, as we have 
seen, has a controlling influence on the amount of debris it 
can transport. A change in conditions which will increase 
the velocity of a stream, other conditions remaining the 
same, will increase its transporting power. The reverse of 
this proposition is also true. 

Movements of the land both of the nature of elevation 
and depression are known to be in progress in many regions 
and, we have good reasons for believing, have at one time 
or another affected every portion of the earth's surface. 
These movements, due to changes in the interior of the 
earth, in many instances affect the surface in such a manner 
as to tilt broad areas. Such tilting furnishes an example 
of the simplest movements of the earth's crust, so far as the 
changes here considered affect the flow of streams. 

When the land is tilted downward in the direction in 
which a stream flows, the velocity of the stream will evi- 
dently be increased and its energy available for corrasion 
and transportation also increased. If, however, the rocks 



STREAM DEPOSITS I 37 

underlying the hydrographic basin of a river are tilted 
downward in a direction opposite to the flow of the stream^ 
its velocity will be decreased, and its corrading and trans- 
porting power lessened. 

It is thus evident that when the land is tilted so as to 
favour corrasion, stream channels will be deepened at a more 
rapid, rate than previous to the change ; and when the tilting- 
is in such a direction as to check the flow of streams, other 
conditions remaining the same, corrasion will decrease and 
deposition may take place and the stream valleys be ag- 
graded. Areas in the course of a stream which are de- 
pressed with reference to adjacent areas will receive deposits 
and be filled until the normal relation of stream bed to 
current has been re-established. An illustration of this 
process is furnished in Southern Washington, at the west 
base of the Blue Mountains, where movements in blocks of 
the earth's crust are producing a depression of an area some 
twenty miles in diameter, with reference to adjacent areas. 
The streams from the Blue Mountains descend steep declivi- 
ties, are swift, and bring large quantities of debris to the 
depressed area. Deposition is there in progress and a gravel 
plain is in process of formation. The streams in crossing 
the area where aggrading is under way, bifurcate much the 
same as on a delta, but the waters unite to form a single 
stream, Walla Walla River, where the area of relative de- 
pression is passed.* 

Influence of Variations in Load on Stream Deposition, — It 
has been demonstrated that a stream of given velocity and 

' I. C. Russell, *' Reconnoissance in Southeastern Washington," U. S. Geo- 
logical Survey, Water Supply a7id Irrigation Papers, No. 4, pp. 23, 24, 1897. 



138 RIVERS OF NORTH AMERICA 

of stated volume has a certain competency to transport 
debris. If the quantity of debris dehvered to a stream ex- 
ceeds its competency, a part is dropped and a part carried 
on. Under such conditions, a selective power is manifest, 
the stream dropping the larger and heavier debris and 
carrying on the smaller or lighter. Changes in velocity, as 
from swift to sluggish reaches in a stream, lead to the drop- 
ping of debris when the current slackens, and a consequent 
aggrading, which tends to give the stream channel a uniform 
slope. If debris is added to a stream already partially 
loaded, the result is much the same as if its velocity was 
checked. If the debris added is in excess of the compe- 
tency of the stream, the coarser material is dropped and the 
finer, up to a certain grade, carried on. 

The principal results of this law are seen where high- 
grade and consequently swift tributaries join a low-grade 
trunk stream. The tributaries may under such conditions 
bring more debris to the main stream than it is competent 
to carry on, and a deposit is made. The result is that an 
obstruction is formed in the main stream at the mouth of 
each high-grade tributary and the waters above are more or 
less completely ponded. Lakes are sometimes formed in 
trunk streams for this cause. Lake Pepin, for example, is 
held in check by debris brought to the Mississippi by Chip- 
pewa River, in excess of the amount the receiving stream 
is able to remove. In the Colorado, as described by Powell, 
rapids due to a similar cause occur just below the mouths of 
several of its tributaries. 

The rate at which streams corrade varies, other conditions 
being the same, with the resistance of the rocks over which 



STREAM DEPOSITS 1 39 

they flow. When the rocks are soft and easily corraded, 
the loads of the streams are increased and their velocities 
checked. In such regions, also, vertical corrasion is fre- 
quently retarded by the occurrence of harder beds farther 
down the streams, while lateral corrasion is still possible; 
the streams then expand in the areas of soft rock and may 
form broad flood-plains. 

The loads of streams are also increased, as previously 
stated, by the action of the wind in bringing sand and dust 
to them. When this occurs, the tendency is much the 
same as when a high-grade tributary delivers more debris 
than the stream can bear away. Trains of sand dunes 
travel over many regions in the direction of the prevailing 
wind. If a train of dunes reaches a river, the load of the 
stream is increased, and may exceed its capacity to transport 
and a dam be formed. The waters may rise above such a 
dam and an increase of velocity be secured which will lead to 
the removal of the obstruction either wholly or in part. In 
many instances, a struggle ensues between the winds bring- 
ing debris to a stream, and the water striving to remove it. 
If the supply of wind-borne debris is sufficient, a dam is 
formed, and may be raised high above the level of the lake 
that it holds in check. The waters of the lake may then 
rise until a balance between inflow and loss by evaporation 
is established, and an enclosed lake, that is, one without an 
outlet, is formed. An example of a water body held in 
check by sand dunes is furnished by Moses Lake in the 
central part of Washington. In this instance the waters 
of the lake escape in part by percolating through the sand 
dunes retaining them. The influence of drifting sand on the 



140 RIVERS OF NORTH AMERICA 

flow of streams is more pronounced in arid than in humid 
regions, for many reasons which will suggest themselves to 
the reader. 

Another way in which the loads of streams are varied, 
is by glacial action. The streams flowing from the ends of 
alpine glaciers, frequently receive greater contributions of 
both coarse and fine material than they are able to bear 
away, and consequently are engaged in filling in their val- 
leys. This is true of every one of the hundreds of glaciers 
in the valleys of the Cordilleran region that have come 
under the writer's notice. Should these glaciers melt, 
the streams flowing from them and now aggrading their 
valleys, would be able to resume the work of excavation, 
and channels would be cut through the deposits of debris 
over which they now meander. 

During the Glacial epoch, when half of North America 
was covered by ice-sheets, the streams fed by the melting 
of the ice were greatly overloaded and their valleys conse- 
quently deeply filled. Now that the ice-sheets have melted, 
the streams are at work in removing the loads they pre- 
viously laid aside. 

Influence of Changes of Climate on Stream Deposition, — 
The elements of climate which exert the most direct and 
important influences on stream deposition are precipitation, 
evaporation, and temperature. 

Of these, precipitation is by far the most important. 
Any change in the amount of rain-fall, or in its distribution 
throughout the year, is at once felt by the streams in the 
region affected. 

Evaporation depends on temperature and on the strength 



STREAM DEPOSITS I41 

of the wind, and tends to diminish the volume of streams 
throughout their entire length. 

Temperature exerts a varied influence. A high mean an- 
nual temperature favours evaporation from the ocean, es- 
pecially, and except under certain local conditions insures 
abundant rain-fall and favours also the growth of vegetation, 
thereby increasing the supply of organic acid available for 
surface water. Warm water charged with organic acid pro- 
motes rock decay and thus favours the preparation of debris 
for transportation. A low temperature, on the other hand, 
not only reverses the conditions just named, but when the de- 
crease is-sufficient to cause the freezing of water, rain changes 
to snow, the ground is frozen, and percolation ceases. The 
storing of the winter's precipitation in the form of snow 
and ice, however, favours stream work and the general degra- 
dation of the land, by concentrating the energy of the streams 
at the time the snow melts. The freezing of water in the 
interstices of rock, as previously mentioned, is one of the 
most powerful agencies tending toward rock disintegration. 
In these and still other ways climatic conditions exert an 
influence either directly or indirectly on stream deposition, 
but at present it will be most profitable to confine attention 
to variations in volume due to changes in supply. 

In general, as is well known, a decrease in the rain-fall of > 
a given region is accompanied by a decrease in the volume of 
the draining streams, and consequently a loss in their trans- 
porting power. The behaviour of streams under such con- 
ditions is materially influenced by the rate at which they 
are supplied with debris. During heavy rains a stream may 
be overloaded and caused to deposit, in spite of its increased 



142 RIVERS OF NORTH AMERICA 

velocity due to greater volume, and the amount of work 
done in a given time is far in excess of that accomplished 
during an equal time when the precipitation is less. 

The influence of variations in precipitation is illustrated 
by the annual change in streams during rainy and dry sea- 
sons. During rainy seasons, more especially in spring in 
temperate latitudes, when the rain causes the melting of 
previously accumulated snow, the streams are swollen and 
heavily charged with debris. They overspread their banks 
and deposit material on their flood-plains. It is during the 
time the streams overflow their banks that the greater 
amount of material is deposited. Much debris is also laid 
down at such times, however, in the stream beds, even 
when the current is swift, and in some instances the less 
heavily loaded water, when much decreased in volume, 
corrades channels through deposits made during high-water 
stage. This may be seen in many wayside rills, which 
spread out in sheets, heavily charged with debris, during 
storms, and make deposits through which the shrunken 
and less heavily charged rills at a later stage excavate 
channels. 

THE GENERAL PROCESS OF STREAM CORRASION AND 
DEPOSITION 

The action of streams in corrading, transporting, and de- 
positing .debris is such a complex process that it is con- 
venient to consider the different phases of their work 
separately. For this reason, an effort has been made in 
this chapter to direct special attention to the manner in 
which streams lay aside their loads during the process of 



STREAM DEPOSITS 1 43 

development that they pass through. The behaviour of 
streams is much like the action of a complex piece of 
machinery, as a watch, for example; changes cannot be 
made in one portion of the mechanism without affecting the 
action of the whole, and necessitating adjustments through- 
out. Considering deposition alone, we find that streams in 
general, in passing from a high to a low grade, make de- 
posits, as where a river leaves its mountain tract and enters 
a valley tract, or passes from a swift to a more quiet reach. 
Streams subject to floods make deposits over the lands they 
inundate during high-water stages, and spread out flood- 
plains. At such times, also, the heaviest deposit is in the 
immediate border of the low-water channel, and natural 
levees are built. A high-grade stream, tributary to a low- 
grade and consequently less swift stream, unless the differ- 
ence in grade is more than counterbalanced by the volume 
of the receiving stream, makes deposits and the waters of 
the main stream are more or .less completely ponded. 
Local overloading from the action of the wind or of 
glaciers produces similar results. 

The debris carried in suspension by streams or rolled 
along their beds is also deposited in lakes as deltas, or dis- 
tributed over their bottom. As lakes in many instances are 
of the nature of expansions of streams, the filling of their 
basins may be considered as a part of the general process of 
stream deposition by which stream channels are aggraded. 
In discussing corrasion it was shown that a stream, at least 
in humid climates, cuts down its channel to baselevel most 
quickly at its mouth, and that the process of deepening pro- 
gresses up stream. The head-waters of a well-developed 



144 RIVERS OF NORTH AMERICA 

stream are steeper than the lower portion of its trunk. A 
general view of stream deposition shows that a similar order 
is followed in the process of stream deposition. When the 
seaward portion of the trunk of a stream has been lowered 
to baselevel, the stream continues to corrade laterally, and 
thus makes it possible for flood-plains to form. As a 
stream continues to widen its channel farther and farther 
from its mouth, the flood-plain follows. If a stream 
is making a delta, its length of flow is increased, and its 
flood-plains and channel are raised by deposition in order 
to furnish the necessary slope. When a stream reaches 
maturity, its plains tract and valley tract are greatly length- 
ened at the expense of the high-grade portions of its course 
in the uplands. The high-grade branches, then, bring 
more material than the trunk stream can bear away, and 
the flood-plains along its sides are raised by the de- 
position of material laid aside. During the upbuilding 
of the flood - plains the stream channel is also raised. 
Hence, for a long time after a normal river has cut down 
its channel in its lower course practically to baselevel, 
building is in progress and the valley becomes filled in with 
abandoned debris. There comes a time, however, when the 
highlands from which the river flows have been lowered 
so that the branches of the main stream are not as swift as 
previously, and the stream is enabled to devote a portion 
of its energy not consumed in the friction of flow to the re- 
excavation of its channel farther seaward. As this process 
is continued, the highest flood-plain is abandoned and new 
ones formed at lower levels, thus giving origin to terraces, 
as will be shown in the next chapter. During this stage of 



STREAM DEPOSITS I45 

a stream's development, as in the preceding stages, changes 
occur, also, in the longitudinal profile of the stream 
throughout its length. 

The manner in which flood-plains are formed and advance 
up stream as the down cutting of the upper portion of a 
stream channel progresses, shows that only an approximation 
to baselevelling is reached during the earlier stages of a 
stream's development. It is after a stream has lowered its 
head-water channels so as to permit of the removal of the 
flood-plains built lower downstream, that what may be 
termed a second approximation to baselevel is normally 
reached. 

PROFILES OF STREAMS 

In a discussion of the succession of changes experienced 
by a stream during its life, consideration should be given to 
the orderly variations in shape that occur in the valley it 
excavates. 

The shape of a valley may be illustrated by two classes of 
profiles; one longitudinal and the other transverse. A gen- 
eralised longitudinal profile of a stream would be what is 
understood as a projection on a vertical plane ; that is, it is 
approximately the profile which the stream would have if it 
flowed in a perfectly straight course from source to mouth. 
Such a profile, together with a sufficient number of cross- 
profiles, would enable one to construct a model of a valley 
showing the actual relations and proportions of its several 
parts. 

The Long it itdhial Profile, — A young stream flowing down 
the surface of a tilted plain, we will assume, will necessarily 



146 RIVERS OF NORTH AMERICA 

have the same gradient as the land which gave direction to 
its current. As such a stream entrenches itself by corrading 
the bottom of its channel, and during the process of cutting 
down to baselevel spreads out flood-plains, which are sub- 
sequently dissected, it will develop a series of profiles to 
suit the various stages of its development. 

An ideal example of the succession of longitudinal profiles 
which a stream makes, may be had by assuming that it 
works in homogeneous rocks throughout its course and is 
not disturbed by changes of climate, the formation of 
glaciers, or other modifying conditions. When the typical 
profile of a young stream and the changes it passes through 
as the stream advances in its appointed task is understood, 
the modifications due to climatic and other disturbances, or 
accidents, as they may be termed, can be readily recognised. 

As has been shown with exceptional clearness by Hicks, * 
corrading streams have curved profiles, the curvature being 
concave upward, while deposits laid down by currents, such 
as alluvial cones, have a reverse curvature, that is, they are 
convex to the sky. The longitudinal profile of a stream 
which is corrading in its mountain tract and spreading out a 
flood-plain farther down its course, must therefore have a 
double curvature — that is, it will be concave in its upper 
course but convex in its lower course. The concave portion 
of the curve is much more conspicuous than the more gentle 
curve due to deposition, and it is frequently stated that the 
profile of a stream is a concave curve throughout its 
length. This, however, can only be strictly true when a 

* L. E. Hicks, " Some Elements of Land Sculpture," in Bulletin of the Geo- 
logical Society of America^ vol. iv., pp. 133-146, 1893. 



STREAM DEPOSITS I47 

Stream is engaged in corrading its channel from source to 
mouth. 

A generalised profile of a stream which is corrading its 
channel throughout is shown approximately in the following 
diagram. It will be noticed that the curvature is compara- 




FlG. 7. Longitudinal Profile of a Young Stream. 

tively great near the source of the stream, but decreases and 
becomes nearly horizontal on approaching its mouth. There 
is a suggestive resemblance between such a profile and 
cycloid curves. As is well known, a cycloid curve is the 
curve of quickest descent for a body moving from a given 
point to a lower one not in the same vertical line. Should 
accurate survey show that streams corrading homogeneous 
rocks actually produce cycloid curves, or the curves of 
quickest descent for their debris-charged waters, it will fur- 
nish another illustration of the wonderful harmony that pre- 
vails in nature. A stream in cutting down its channel to 
baselevel must evidently reach that limit first at its mouth, 
and will then continue to deepen its bed progressively up 
stream. If this operation should be allowed to go on with- 
out deposition and the formation of flood-plains, the result 
would evidently be the flattening of the curved profile from 
the mouth of the stream to its source at the same time that 
the elevation of the stream channel above sea-level was 
lowered progressively and in an increasing ratio from mouth 
to source. Corrasion, however, is accompanied by sedi- 



148 RIVERS OF NORTH AMERICA 

mentation. In young streams, corrasion may occur through- 
out their courses, but as soon as their mouths are lowered 
to baselevel, deposition begins and progressively advances 
up stream. The longitudinal profiles of most streams result 
from both corrasion and sedimentation, and have a double 
curvature. Corrasion is more active in the mountain tract 
than in the valley and plains tracts, and until these divisions 
are obliterated by advancing age, the profile of a stream is, 
in part, due to corrasion and in part to sedimentation. 
With advancing age the portion of the curve due to deposi- 
tion advances up stream at the expense of the steeper 
portions of the profile where corrasion is still in progress. 
There comes a time in the development of a stream, how- 
ever, when this advance is checked, and when the flood-plain 
deposits begin to be dissected ; the swing is then the other 
way, and the portion of the profile due to corrasion is 
lengthened and progresses toward the mouth of the stream. 
In old age the profiles of streams become flattened and ap- 
proach more and more nearly a straight line, but probably 
never reach that condition. 




Fig. 8. Successive Changes in the Profile of a Divide Owing to Corrasion and 
Weathering : Vertical Scale Exaggerated. 

The heavy broken line indicates the profile of an uplift as it might appear had there been no 
erosion ; the smaller broken lines show weather-curves ; the dotted lines, successive cor- 
rasion curves ; and the solid curved line below, the profile of the resulting old-land surface. 

In the above diagram an attempt is made to show quali- 
tatively the successive changes that the profiles of streams 
pass through from youth to old age. In the case assumed, 



STREAM DEPOSITS 1 49 

two streams flow in opposite directions from a common 
divide, and are so nicely balanced against each other that 
the divide has been lowered in a single vertical plane. The 
concave curvature of the profiles in their upper courses in- 
creases during early youth, reaches its maximum when the 
streams are mature, and then decreases with advancing age. 
On account of the exceedingly gentle concave curves due to 
deposition, it is impossible to represent them on the scale 
here used. 

When the profiles of oppositely flowing streams meet at 
the crest of a mountain range, there should be, if no modify- 
ing conditions intervene, a sharp divide, as is indicated in 
Fig. 9. On some mountain crests this condition is very 
nearly reached. As one follows up a stream and approaches 




Fig. 9. Ideal Profile of a Divide between the Head-W^aters of Two Opposite- 
Flowing Streams : Vertical Scale Exaggerated. 

its ultimate source, the rate of corrasion progressively di- 
minishes, for the reason that the water supply becomes 
smaller and smaller. The rocks, however, are everywhere 
exposed to the denuding agencies of the air, namely, rain, 
wind, frost, etc., and at the heads of drainage lines the 
action of these agencies is in excess of stream corrasion, and 
convex curves due to weathering modify or replace the con- 
cave curves due to stream action. Unless the rocks on a 
divide between two drainage systems which head against 
each other are unusually resistant, and maintain angular 
forms as they weather, the concave profile leading up to the 



150 RIVERS OF NORTH AMERICA 

divide from either side changes to convex curves before 
uniting.* The usual profile in such instances is shown in 
the lower curves in Fig. 8. 

Cross-Profiles. — The cross-profiles of stream-cut valleys 
change in the same locality with the age of the stream, and 
are modified by the weathering of the valley sides, the 
texture of the rocks, etc. If, as above, we conceive of a 
valley being cut out of homogeneous rocks and ascertain 
what changes in its cross-profile at a given locality will result 
from stream action and weathering, the modifications due 
to other causes may be more easity recognised. 

A young and rapidly corrading stream working in moder- 
ately hard rocks produces a gorge or canyon with steep 
sides. The cross-profile of such a gorge is markedly V- 
shaped, except that the bottom of the V is slightly 
rounded. The width of the concave bottom of the trench 
varies with the size of the stream. If the stream is work- 
ing in hard rocks the sides of the trench cut by it rnay be 
vertical. 

As a stream advances with its task of cutting down its 
channel to baselevel, its energy available for corrasion is 
more largely exerted in the direction of broadening its val- 
ley. The cross-profiles of the valleys of old streams be- 
come broadly U-shaped. The valleys of streams where an 
approximation to baselevel has been reached, or when flood- 
plains are being formed, generally have flat bottoms with 
more or less flaring sides. The cross-profiles then resemble 
more or less closely the figure which would be obtained by 

' L. E. Hicks, ** Some Elements of Land Sculpture," in Bulletin of the Geo- 
logical Society of A vi erica, vol. iv., pp. 133-146, 1893. 



STREAM DEPOSITS I5I 

breaking a plate straight across. That is, the bottom is a 
horizontal line bordered by ascending lines. The graceful 
double curves in the profile on each side of an aggrading 
stream have already been referred to. 

As a stream advances in age, the cross-profile at a given 
locality gradually changes from a V-shape to a U-shape, 
and then to a -^ — /-shape. In extreme old age the bottom 
becomes greatly broadened with reference to the height of 
the sides. 

The slope of the sides of a valley, whatever its age, de- 
pends on the texture of the rocks and on weathering. In 
hard rocks the slopes are steeper than in soft rocks. If the 
rate at which a stream deepens or widens its valley is rapid 
in reference to the rate of weathering, the sides will be 
steep, but if the reverse is the case the slopes will be gentle. 
It is thus evident that the character of the cross-profile of a 
stream-cut valley depends largely on climate, on rock text- 
ure and rock structure, on relative rate of corrasion and 
weathering, and on the stage in development that the stream 
has reached. 



CHAPTER VI 

STREAM TERRACES 

A TERRACE may be defined as a step-like area with a 
nearly even and approximately level surface, bounded 
on one margin by an ascending and on the other by a 
descending slope. A stairway may be considered as an 
example of a series of terraces bounded by vertical escarp- 
ments. In nature there are many departures from the reg- 
ularity in form implied in the above statements, due in part 
to the conditions under which they were made, but more 
commonly to subsequent changes. The surface of a terrace 
is frequently uneven, and cut across by rill-channels and 
gullies, or talus slopes and landslides may encumber it. 
The bounding slopes may be steep, or depart but slightly 
from the horizontal. A cross-profile of a river valley with 
terraces on each side is shown in the following diagram. 




Fig. io. Ideal Cross-Profile of a Terraced Valley. 

This figure is intended simply to illustrate the general char- 
acteristics of stream terraces, and not to indicate the precise 
conditions which the student may expect to find when he 
supplements his reading by cross-country tramps. 

152 



STREAM TERRACES I 53 

Terraces of this general character, from a few feet to 
several rods broad, may frequently be traced for many miles 
on each border of a river valley. In numerous instances 
several terraces one above another with various intervals be- 
tween may be recognised on the same slope. They follow 
all of the windings of the valleys, sweeping about prominent 
bluffs and into adjacent embayments in broad, beautiful 
curves. Much of the charm alike of sheltered dells and of 
broad valleys is frequently due to the symmetrically curving 
lines formed by the terraces on the bordering slopes of the 
adjacent uplands. This is true more especially of the valleys 
of the Northern Appalachians and New England and 
thence westward through the vast areas formerly occupied 
by glaciers. Many of the valleys in the mountains of 
the Cordilleran region are also terraced in a remarkable 
manner. 

River valleys, as we know, have been excavated by the 
streams flowing through them, and it is at once evident 
that the terraces beautifying their sloping sides must, in 
most instances, be due to the same agency. Another obser- 
vation confirming this conclusion, is that the terraces are 
not horizontal when followed in the direction of their lengths, 
but have a gradient similar to that of the stream flowing 
through the bottom of the valley in which they occur, but 
not precisely coinciding with it. 

The fact that stream terraces are not horizontal in the 
direction of their lengths serves to distinguish them from 
similar topographic forms made by the waves and currents 
of lakes or of the ocean. The surfaces of standing water- 
bodies are horizontal in the every-day sense of the term, 



154 RIVERS OF NORTH AMERICA 

and the terraces made by such water bodies on the land 
confining them are also horizontal. 

The presence of terraces on the borders of stream-cut 
valleys suggests that they owe their origin to the processes 
of corrasion or of deposition which characterise the work of 
streams. The study of the topographic forms under con- 
sideration has shown that they may be due to either of these 
processes, or to their combined action. Certain stream ter- 
races have been formed by excavation, others are the result 
of deposition, while still others owe their existence to a 
combination of the two processes. We might classify them 
as cut terraces, built terraces, and cut-and-built, or com- 
pound, terraces. Such a classification has but little signifi- 
cance, however, unless the relation of the terraces to the life 
histories of the streams which gave them origin is under- 
stood. 

When the life histories of streams are reviewed, and the 
modifications in their normal development due to climatic 
changes and to secular movements in the earth's crust are 
considered, it will be found that there are three principal 
causes which lead to the origin of terraces. These are : ist. 
the normal changes in a stream valley due to the successive 
processes of corrasion, flood-plain building, and re-excava- 
tion ; 2d. climatic changes which cause variations in the 
volumes of streams or lead to excessive deposition for a 
time, and the re-excavation of the partially filled valleys; 
and 3d. oscillations in the land which vary the rate of cor- 
rasion and of deposition. Let us consider these three 
methods in the order named. 

Origin of Terraces during the Process of Normal Stream 



Plate VIM. 




Fig. a. Terraces on Fraser River, British Columbia. 
Showing post-glacial re-excavation, (Photograph by Geological Survey of Canada,) 




Pig. B. Terraces in Connecticut Valley, near Bellows Falls, Vermont. 
(Photograph by C. H, Hitchcock.; 



STREAM TERRACES I 55 

Development, — In discussing the combined process of stream 
corrasion and deposition, when not seriously modified by 
climatic changes or movements in the crust of the earth, 
it was found that a river in flowing from highlands to the 
sea first cuts down its channel to baselevel at its mouth 
and then lowers it progressively up stream, but during 
its early life makes only an approximate adjustment to 
the level of the receiving water-body. Succeeding the 
first stage of excavation and following it progressively 
up stream, the valley is aggraded. This combined pro- 
cess is checked when the head branches of the river 
no longer supply more debris than the trunk stream can 
carry away, or, less commonly, when the course of the river 
is lengthened by the formation of a delta. The stream then 
begins to excavate a channel through the flood-plain pre- 
viously formed. When this process of re-excavation begins 
the stream is usually meandering in broad curves over a 
flood-plain. As the stream deepens its channel and sinks 
below the level of the flood-plain, it retains its windings; 
although the accelerated velocity of the stream may tend 
appreciably to straighten its course. When the stream 
lowers its channel, portions of the original flood-plain are left 
as terraces on the sides of the valley. At the time a stream 
begins to deepen its channel, it may, in one portion of its 
course, be in the centre of its flood-plain, and will then leave 
a terrace on each side, or may flow on one side or the other 
of its valley, and therefore leave a terrace only on one border 
of its course. The stream may then broaden its channel, 
and spread out a second flood-plain in the valley excavated 
through the previously formed deposit. 



156 RIVERS OF NORTH AMERICA 

A stream in flowing down a flood-plain, it will be remem- 
bered, makes not only short bends, but broad sweeps which 
carry it from one side of its valley to the other. The short 
bends are made during periods of time measured by tens or 
hundreds of years, while the great migrations from side to 
side of a broad valley require thousands of years to com- 
plete a single swing. The short bends which combine to 
make much greater curves have been referred to in the case 
of the Mississippi, and may be readily recognised on any 
good map of that river. While a stream is deepening its 
channel in a broad alluvial plain and building a second 
flood-plain at a lower level, the down-cutting, between the 
time it leaves one border of its valley, migrates to the other 
side, and returns, may be so great that on its return it will 
be flowing at a sufficiently lower level to prevent its re- 
flooding its previously formed flood-plain. When this hap- 
pens, and the stream in its migrations does not swing back 
to its previous position, a portion of the flood-plain is left 
and forms a terrace. Successive terraces may be left at 
lower and lower levels by a continuance of this process. 

A cross-section of a valley terraced in the manner just de- 
scribed would present the features shown in the following 
diagram. Each terrace is a portion of a flood-plain deposit^ 




Fig. II. Ideal Cross-Section of a Partially Filled Valley with Terraces Left 
during Re-Excavation. 

and the highest in the series is the oldest. The material 
forming the superficial portion of the second terrace from 



STREAM TERRACES 1 57 

the top has been removed by the stream, and re-deposited 
as a portion of the second-formed flood-plain, and this 
process has been repeated also in the case of the third 
terrace. 

In the normal development of a stream after the stage in 
a certain portion of its course, indicated in Fig. 11, is 
reached, the stream will continue to deepen its channel, and 
may cut into the rock below the flood-plain deposits. This 
stage in the process is illustrated by the cross-section shown 
below. Should subaerial erosion remove the alluvial ma- 
terial indicated by dots in the diagram, a rock terrace would 
be left. If stream development progresses and a second 






Fig. 12. Ideal Cross-Profile of a Partially Alluvial- Filled Valley Re-Excavated 
to below its Original Depth. 

approximation to baselevel is made, all of the alluvial ma- 
terial and a portion of the rocky floor on which it rests may 
be removed. 

Other ways in which normal alluvial terraces might be 
formed have been cited by Dodge.* Suppose that a stream 
whose load is slightly in excess of its carrying power ac- 
quires by capture the head-waters of another stream, as will 
be considered later. In the district thus acquired there 
might be an excess of carrying power over load ; if such was 
the case, the capturing stream would have its carrying power 
increased without a corresponding increase in load, and 

' R. E. Dodge, *' The Geographical Development of Alluvial Terraces," in 
Boston Society of Natural History, Proceedings, vol. xxvi., p. 263, 1S94. 



158 RIVERS OF NORTH AMERICA 

therefore be able to deepen its channel in previously de- 
posited alluvium, and terrace it. 

Again, as cited by Dodge, a stream which had been work- 
ing in soft rocks might cut down into hard rocks underneath 
the soft ones. The effect of such a change on the head- 
waters of a stream would be to decrease its load and enable 
it to corrade in its alluvial tract. Hence, without varying 
in volume a stream might be able to terrace an alluvial plain 
formed while it was previously removing soft rock. 

Thus in several ways, or as a result of the combined in- 
fluence of two or three normal variations in streams, alluvial 
terraces might result. These processes of terrace-making, 
however, are slow, and the topographic forms resulting may 
be greatly modified or even obliterated by subaerial denuda- 
tion as fast as they appear. These processes, also, are a 
part of a larger process, i. e., cutting to baselevel, which in- 
sures the ultimate destruction of the topographic features 
referred to. For these reasons the methods of terrace- 
making just considered have received but little attention, 
and their results are difficult to recognise. And, besides, 
other methods of terrace-forming are apt to produce such 
conspicuous results that the terraces due to what has been 
termed the normal stream development are usually masked 
or obliterated. 

Terraces Due to Climatic Changes. — In considering the 
various influences of changes of climate on stream deposi- 
tion, it was shown that heavy rains may cause the tributaries 
of a stream to bring to the main channel more debris than 
can be removed, and deposition takes place. In a similar 
way a secular change of climate producing an increase in 



STREAM TERRACES I 59 

precipitation, might lead to the filling, especially of low- 
grade river-valleys, and the raising of the flood-plains 
throughout all of their lower courses. 

A climatic change which would admit of the birth and 
growth of glaciers on the higher portions of a mountain 
range, previously deeply stream-sculptured, would lead to 
the overloading of the streams below the glaciers and the 
thickening and broadening of the flood-plains throughout 
their lower courses. 

Climatic conditions favourable for the birth and growth 
of glaciers are usually, and probably always, accompanied 
by increased precipitation and decreased evaporation. Thus 
for several reasons the occurrence of a glacial epoch like 
that in late geological time, when one-half of North America 
was occupied by ice-sheets, would favour the filling of pre- 
existing valleys with debris. When the climate experienced 
a reverse change and the glaciers melted, the draining 
streams would for a time be still more deeply flooded, and 
additional quantities of debris carried from high to low 
regions. If a warmer and drier climate should succeed a 
glacial epoch, the streams, no longer heavily loaded, would 
begin the task of removing the debris deposited in their 
valleys during the preceding time of overloading. As the 
streams deepened their channels in the alluvium previously 
deposited, portions of the flood-plains left intact would ap- 
pear as terraces, and the elevation of their surfaces would 
record the depth to which the valleys had been filled with 
debris. 

This process of removing the accumulations of debris 
clogging a valley might be accompanied by the formation of 



l6o RIVERS OF NORTH AMERICA 

terraces at lower levels, according to the laws, cited above, 
governing the normal development of streams. As will be 
shown later, however, a still more potent agency in the 
formation of the lower terraces would be climatic changes 
and periodic elevation of the land. 

Should several glacial stages occur with intervals of milder 
and less humid climatic conditions intervening, it is evident 
that the terraces resulting from the trenching of the first- 
formed flood-plains might be obliterated by subsequent de- 
position, and the surface of the debris in the valleys be carried 
higher than during the first ice invasion ; or the valleys, cut 
in the first-formed flood-plain, might be only partially re- 
filled, and when excavation was renewed, lower terraces 
would be formed. 

The conclusion that glacial conditions would lead to 
the filling of pre-existing valleys downstream from alpine 
glaciers or about the margins of piedmonts and continental 
ice-sheets, and portions of these deposits be left as terraces 
when corrasion was resumed, is sustained by an abundance 
of examples throughout the northern portion of the United 
States and Canada. In the valleys in this region terraces 
excavated in m^aterial deposits by glacial streams are mag- 
nificently displayed. On the head-waters of Columbia River 
in Washington and Idaho, terraces of the nature here con- 
sidered are perhaps as well developed, and their history as 
easily read, as in any other portion of the continent. The 
great canyon of Snake River, the principal tributary of the 
Columbia, was excavated to its present depth, — four thou- 
sand feet throughout a considerable portion of its course, — 
previous to the Glacial epoch. During that epoch, glaciers 



STREAM TERRACES l6l 

existed in the more elevated valleys and about the sum- 
mits of the mountains of Idaho. The branches of the 
Snake were flooded, and brought such quantities of debris 
to the canyon of the main stream that throughout hun- 
dreds of miles of its course it became filled to a depth of 
three hundred and sixty feet. When the glaciers passed 
away and the streams were no longer supplied with debris 
by them, and still more effectually when a mild and but 
moderately humid climate prevailed, the streams were 
-enabled to attack their flood-plains and cut valleys and 
canyons through them. Snake River has now removed by 
far the greater portion of the coarse gravel and boulders 
that formerly occupied its canyon, and has resumed the task 
of deepening its channel in the hard rock beneath. Episodes 
similar to that just referred to in the history of Snake River, 
but with various minor modifications, occurred in the lives of 
tens of thousands of streams not only in the northern part 
of North America, but as far south as the Gulf of Mexico 
and also in the Rocky Mountains and Sierra Nevada, as a 
result of the climatic change to which the Glacial epoch was 
due. 

In studying the effects of changes in climate on the be- 
haviour of streams, the fact should be borne in mind that 
such changes, although by reason of the comparatively brief 
time during which man has taken account of secular vari- 
ations in atmospheric phenomena they are commonly 
considered as exceedingly slow in their occurrence and em- 
bracing but a moderate range, appear relatively rapid and 
of well-marked amplitude when such periods of time as are 
involved in geographic cycles are studied. Many of our 



1 62 RIVERS OF NORTH AMERICA 

rivers, as, for example, the Susquehanna, Mississippi, and 
Columbia, were far advanced in their development before 
the beginning of the Glacial epoch. The time that has 
elapsed since the melting of the continental glaciers on the 
head-waters of these rivers is but a small fraction of the cur- 
rent geographic cycles. Many annual climatic changes, as 
is well known to everyone, occur while even a meadow 
brook undergoes but slight modifications ; in a similar way^ 
as is well known to geographers, many secular changes in cli- 
matic conditions may take place during the life history of a 
great river. 

Terraces Due to Elevation of the Land. — The manner in 
which a stream carries on its work, it will be remembered^ 
is controlled in an important way by declivity. Conse- 
quently, changes in the elevation of the land must have a 
direct bearing on the history of the streams draining an 
area thus affected. The movements in the earth's crust 
referred to are known to have modified the surface slopes 
throughout large areas, and frequently to be of the nature 
of a tilting of the land. Other movements occur, but at 
present let us consider simply the effects of the tilting of a 
region drained by a large river on the problem of terrace- 
making. 

A tilting of the rocks which decreases the gradient, and 
consequently the velocity, of a stream, other conditions re- 
maining the same, will favour deposition, and may lead to 
the partial or complete filling of its previously formed valley. 
The flood-plain deposits would then increase in thickness 
and become broader at the surface. In other words, a de- 
crease in velocity favours the process of aggrading. 



STREAM TERRACES 163 

If the region drained by the Connecticut, for example, 
be considered as a plane gently inclined southward, and to 
be affected by a movement in the earth's crust which de- 
creases the gradient of the river, the depression of the land 
being least at the south and progressively increasing north- 
ward, — that is, the hinge-line, so to speak, on which the 
tilted block of the earth's crust moves, being situated near 
its southern margin, — the main trunk of the river would have 
its current slackened, and its transporting power diminished, 
while the gradients of the branches of the river coming in 
from the east or west would be but slightly affected. The 
direct result of such a change would be to favour deposition 
in the main valley and to a less extent in its branches. 

If, after such a change of grade as has been postulated, 
when the valley of the Connecticut has become deeply filled 
and a broad flood-plain spread out, we imagine the land to 
remain stationary, the branches of the main river would cut 
down their channels, thus decreasing their velocities and 
diminishing the amount of debris carried annually to the 
main valley. When this stage had been reached, the Con- 
necticut would begin to cut a channel through its previously 
formed flood-plain, as in the case of normal stream develop- 
ment already considered. 

In case the inclined plane drained by the Connecti- 
cut should experience a reverse movement after its valley 
had become deeply filled — that is, if elevation should 
occur, the hinge-line retaining its former position, — the 
gradient of the main stream and of all its branches flowing 
southward would be increased, while the lateral branches 
would be but little affected. The increased gradient of the 



164 RIVERS OF NORTH AMERICA 

main stream would give its waters greater velocity, thus 
favouring corrasion at the expense of deposition, and a 
channel would be cut through the previously formed flood- 
plain. Portions of the flood-plain not removed would re- 
main as terraces. Imagine the re-elevation at the northern 
border of the tilted area to be one hundred feet, and to de- 
crease to zero at the hinge-line at the south. The result 
would be acceleration of velocity in the extreme head 
branches flowing southward ; this might cause them to bring 
more debris to the main stream than it could transport, but 
the branches from the east and west being but slightly 
affected, the more probable result would be the deepening 
of the bed of the main stream throughout. The river 
would excavate a channel through its previously formed 
flood-plain, leaving portions of it on either side of the valley 
as terraces. When the river, after adjusting itself to the 
new conditions, began to broaden its channel and spread 
out a second flood-plain, it would be flowing a hundred feet 
below its former bed- in the upper portion of its course, but 
this difference would gradually decrease downstream and 
become zero where the hinge-line was crossed. The south- 
ward or down-stream slope of the surface of the old flood- 
plain, portions of which remain for a time as terraces, would 
therefore be greater than the slope or gradient of the 
readjusted stream. This postulated case thus furnishes an 
explanation of the fact that when a number of stream ter- 
races occur on the border of a valley, they are not only not 
horizontal, but have different gradients. The gradient of 
every stream terrace is determined by the gradient of the 
parent stream at the time it was formed. 



STREAM TERRACES 165 

The terraces originating in the several ways thus far con- 
sidered consist of alluvium, which was deposited in a pre- 
viously formed river valley, and the surface of each terrace 
is a portion of a flood-plain. In cross-section, such terraces 
would have the characteristics shown by the diagram on 
page 156, introduced in connection with the discussion of 
what are termed normal terraces, and would be cut through 
or finally removed during subsequent stream development 
in the manner already described, unless subsidence carried 
them below baselevel. In the discussion just presented, we 
have assumed a river valley to have been deeply filled with 
alluvium previous to the elevation of the land which enabled 
the stream to deepen its channel. This assumption is not 




Fig. 13. Ideal Cross-Section of a Valley with Terraces Cut in Solid Rock and 
Covered with Alluvium. 

necessary, however, and numerous instances might be cited 
where terraces in solid rock have resulted from accelerated 
corrasion due to periodic uplifts. Imagine a stream like the 
Connecticut to have broadened its valley and spread out a 
flood-plain, and then an elevation to take place as before. 
Accelerated velocity may enable the stream to lower its bed 
so as to cut through the flood-plain deposits and into the 
rocks beneath. A broadening of the new channel may then 
occur, and renewed elevation allow the process to be re- 
peated. With each upheaval the stream cuts deeper into 
the rocks, leaving each time a terrace of solid rock with a 
sheet of alluvium on its surface. The characteristic features 



1 66 RIVERS OF NORTH AMERICA 

of a cross-section of such a terraced valley are shown in 
the ideal diagram, Fig. 13. As the excavation of solid rock 
is normally a slow process, the sheet of alluvium covering 
the terraces would be apt to be removed by rain, rills, etc., 
and rock terraces but scantily covered or without debris 
be exposed. 

The formation of terraces during what has been termed 
the normal development of a stream — that is, when changes 
of level have not occurred, and climatic variations, etc., have 
not materially affected its volume, velocity, or load — is an 
extremely slow process, and, as previously stated, it is prob- 
able that atmospheric agencies under most climatic condi- 
tions would destroy the terraces as fast as formed. For this 
and other reasons it is believed that most of the terraces on 
the borders of stream-cut valleys are records of climatic 
changes which caused excessive deposition in low-grade 
valleys followed by a period of erosion ; or are due to land 
oscillation. 

Bottom Terraces, — Still another variety of terraces is 
formed by streams by deposition when their bottom loads 
are small. When their bottom currents are underloaded, as 
we may term the condition here referred to, the material is 
carried forward like a wave, in the manner in which a ripple 
in sand is produced under the influence of a wind- or water- 
current, and deposited with a steep escarpment facing down- 
stream. These bottom terraces have broad, gently ascending 
surfaces in the direction of the flow of the current, and 
steep escarpments, facing downstream, and trend in general 
at right angles to the flow of the water, but are usually 
lobed on their lower margins. Such terraces or broad 



STREAM TERRACES 1 67 

ripples may be seen in process of growth in many clear 
streams which have moderate bottom-loads of coarse sand 
and gravel. They are frequently several feet or even yards 
broad, with escarpments from a few inches to a few feet 
high. Although of minor importance when considered in 
connection with associated stream-made topographic forms, 
yet under special conditions, as when a broad stream is mov- 
ing debris over a gentle slope, they might become relatively 
conspicuous if the stream should be diverted. There is a 
gradation between bottom terraces of the nature just con- 
sidered and delta terraces which would repay investigation. 

Delta Terraces and Current Terraces, — In the discussion 
of deltas in a previous chapter, it was shown that they are 
formed where streams deposit their loads on entering still 
water. Now, streams sometimes expand and have sluggish 
currents so as to simulate lakes. When this happens a 
tributary stream freighted with debris may drop a por- 
tion of its load and build up a delta-like deposit at its 
mouth. 

The most favourable conditions for this process are when 
low-grade, sluggish rivers extend into embayments on their 
borders, as the mouths of tributary valleys, and a stream 
from the tributary valley brings in sediment. A lowering 
of the main stream after such a delta has been formed 
would leave it as a terrace. Such structures have been 
termed delta terraces by Edward Hitchcock.^ A section of 
a delta terrace would reveal a series of inclined beds, as 
shown in the diagram on page 126, and possibly the upper 

^ " Illustrations of the Earth's Surface," Smithsonian Contributions to Knoiv- 
ledge, vol. ix., pp. 32-34, 1857. 



1 68 RIVERS OF NORTH AMERICA 

and lower members of a typical delta built by a high-grade 
stream as well. The surface of such a delta terrace would 
have a slope corresponding with the grade of the supplying; 
stream. 

The current of a river washes its banks in much the same 
way as the currents in lakes wash their shores. The study 
of the action of lake currents has shown that they bear 
along debris, and drop it in part so as to form what are 
known as built terraces. The current, especially of a broad 
river, behaves in much the same manner. Debris brought 
by tributary streams, or derived from localities where the 
river is corrading its banks, is carried down stream and 
may be deposited adjacent to the shore, so as to form a 
built terrace. A subsidence of the waters would leave the 
terrace exposed. Its surface would slope gently toward the 
stream, and, as in the case of all river terraces, would have 
a gradient, when followed along the valley on the side of 
which it was formed, corresponding with the surface gradient 
of the building stream. In cross-section such a terrace 
would reveal the structure characteristic of built lake- 
terraces, the general features of which are shown in the 
following ideal diagram. The slope rising above such a 




Fig. 14. Ideal Cross-Section of a Current-Built Terrace. 

terrace may be the valley side, produced by stream corrasion 
and weathering, or be a steeper slope due to lateral corra- 



STREAM TERRACES 1 69 

sion of the current and correspond more nearly with the 
'* sea-cHff " above a lake terrace. 

The waves and currents of a broad river may lead to cor- 
rasion along its shores in the same manner as in a lake, and 
cut terraces result. A miniature example of this is shown 
in Fig. F, Plate II. 

So far as the present knowledge of stream terraces allows 
one to judge, it does not appear that those built after the 
manner of lake terraces, as just described, are common. In 
fact, delta terraces and current terraces, as they may be 
termed, depend for their origin on a delicate balancing of 
conditions which apparently is seldom reached. 

Delta terraces and current terraces formed on the sides of 
streams are of interest, as they constitute a group, although 
small and of minor importance, which may be designated as 
built terraces in distinction from other stream terraces which 
are due to both deposition and excavation, or to excavation 
alone. The downward slope bordering the nearly flat sur- 
face of a built terrace is due to deposition ; in the other 
varieties, this slope is produced by excavation. 

Glacial Terraces, — The terraces built by streams con- 
jointly with glaciers need not claim much attention at this 
time, since they derive their greatest interest from their 
connection with the ice-bodies about which they are formed. 
When a glacier, however, or perhaps more frequently a stag- 
nant ice-mass, occupies a valley, streams sometimes bring 
gravel, sand, etc., and deposit it along the margin of the ice 
so as to give a level floor to the space intervening between 
the ice and the valley border. After this space has been filled 
to a greater or less depth and the ice melts, the deposit re- 



IJO RIVERS OF NORTH AMERICA 

mains as a terrace.' Such terraces have approximately level 
surfaces, are composed of current-bedded gravel and sand, 
and perhaps certain occasional boulders or angular rock- 
masses, but do not exhibit the arrangement of coarse and 
fine material characteristic of flood-plains; and, besides, 
their down-stream gradients are markedly different from 
those of true stream-terraces. 

Relative Age of Terraces, — When stream terraces occur 
one above another on the side of a valley, the highest in 
the series is usually the oldest. But exceptions to this 
rule may occur, as when changes of level lead to the build- 
ing of delta or current terraces on the surface of previously 
formed terraces. Again, a valley in which a terrace has 
been cut in solid rock might become filled with alluvium 
so as to bury the terrace, and re-excavation again bring it to 
light, and form another terrace at a higher level; the lower 
terrace would then be older than the one above it. 

In a series of alluvial terraces in which the highest is the 
oldest, each one or each pair, if fragments of the same flood- 
plain are left on each side of the valley, is a remnant of 
a flood-plain, and the material in the highest terrace is 
younger than the main portion of each lower terrace ; but the 
surface portion of each terrace was worked over and re- 
distributed at the time the flood-plain of which it is a part 
was formed, and hence may be said to be younger than the 
material in each higher terrace. 

' The terraces here referred to have been termed " kame terraces" by R. 
D. Salisbury, Geological Survey of New Jersey, Annual Report for i8gj, pp. 
I55» 156. Similar topographic forms were previously termed "moraine ter- 
races" by G. K. Gilbert, U. S. Geological Survey, MonograpJis, vol. i., p. 8l, 
1890. 



STREAM TERRACES I7I 

Other Terraces, — Terraces similar to those formed by 
streams originate in other ways, and it is important that 
the student of geography and geology should be able to dis- 
tinguish those which owe their origin to one series of agencies 
from those belonging to other categories. 

Cut terraces in rock or in loose material are a characteris- 
tic feature of lake and ocean shores, as are also delta and 
current terraces. In nearly all of their main features, these 
terraces are similar to stream terraces except that they are 
essentially horizontal when traced in the direction of their 
length. Movements in the earth's crust, however, may tilt 
a previously horizontal terrace so as to give it a gradient 
closely approximating to the normal slope of a stream 
terrace. A similar tilting of the land might affect a river 
terrace so as to alter its gradient and perhaps make it hori- 
zontal. In cases of this sort associated topographic features 
would usually furnish the best clue to the true history. 
River terraces are formed in comparatively narrow valleys, 
while lakes may occupy valleys of any shape. Lake ter- 
races are usually accompanied by escarpments which rise 
above them, termed *' sea-cliffs " ; these may or may not be 
characteristically different from the corresponding slopes 
above river terraces. The normal lake-terrace is either cut- 
and-built — that is, it is a shelf made by excavation with a 
current-built covering on its surface, — or may be entirely 
a deposit formed by construction. Stream terraces do not 
usually have this structure, but yet may have it. As may be 
judged, the tests just suggested might not lead to definite 
conclusions. In fact, in the case of abandoned stream- and 
lake-terraces, that is, when a former lake basin has been 



1/2 RIVERS OF NORTH AMERICA 

emptied and perhaps has a stream flowing through it, or 
when a former stream valley is no longer a line of drainage, 
it is frequently difficult to satisfactorily determine their 
origin. In such an instance, if the former topography is 
not greatly altered, it may be possible to work out the 
history of the changes that have occurred and to construct 
a map of the country as it existed when the terraces were 
formed, and thus be able to decide whether flowing water 
or bodies of still water were responsible for the terraces. 

River terraces frequently make a direct connection with 
lake terraces so that the place of junction may be difficult 
to determine. The main difference to be looked for in such 
an instance would be a change from horizontality to an in- 
clination in the surfaces of the terraces where it passed from 
the lake valley into the stream valley. In a tilted or other- 
wise disturbed region, where both lake and stream terraces 
occur, a difference in their gradients may still be recognis- 
able, and assist in their discrimination. 

Instances occur, however, in disturbed and eroded re- 
gions, where only fragments of terraces remain, when it 
is practically impossible to tell whether they record lacustral 
or stream conditions. River terraces also pass into terraces 
made about the borders of estuaries and on ocean shores. 
Here, again, when disturbances occur and the estuaries are 
emptied of their water and the streams have been diverted, 
difficulties in interpreting the records might arise. The 
sedimentary deposits made on the floor of the estuaries and 
the evidences of life buried in them, as well as the fossils in 
the terraces themselves, might here furnish assistance. The 
shells in river terraces will be fresh-water or land species; 



STREAM TERRACES 1 73 

while those in the estuary sediment and terraces will be, in 
part at least, such as inhabit brackish or saline water. 

Terraces also result from the weathering of the outcrops 
of alternating hard and soft strata. When the strata are 
horizontal or but slightly inclined, the hard beds may stand 
out as shelves or terraces, as, for example, in the sides of a 
valley cut in stratified rocks. The downward slope of such 
a terrace is the exposed edges of hard layers, and may be 
steep or gentle according to climatic and other conditions; 
the slope rising above the terrace is formed of the edge of 
the weak strata above the terrace-making layer, and is 
usually a gentle slope unless the layer is thin and another 
hard terrace-making layer occurs just above. The arrange- 
ment of resistant and weak strata in the case of these 
terraces of differential erosion usually makes it easy to dis- 
tinguish them from river terraces. There is an absence, 
also, on the terraces of this nature, of stream deposits, 
and this negative evidence might assist in the diagnosis. 
Weathered debris falling on the surface of a terrace of 
differential erosion may simulate stream deposits, however^ 
and lead to erroneous conclusions. 

Fractures in the rocks along which differential movement 
of the sides has taken place, producing what are known as 
faults, may also give origin to topographic forms of a terrace- 
like character, but these are usually irregular, with reference 
to both horizontal and vertical planes, and in most instances 
are easily distinguishable from stream or other terraces. 

Landslides also produce terrace-like forms, but these are 
seldom continuous for considerable distances, and are usually 
so irregular and bear such relations to the slopes from which 



1/4 RIVERS OF NORTH AMERICA 

the fallen blocks descended, that their origin may usually 
be readily determined. When a landslide occurs it fre- 
quently happens that the displaced material acquires a sur- 
face slope toward the place from which it came. This 
backward slope frequently produces basins in which lakes 
and swamps occur, thus furnishing additional evidences 
bearing on the origin of the terrace-like forms produced.* 

Terraces due to still other causes might be enumerated 
and the means for their discrimination indicated, but I be- 
lieve those most nearly simulating stream terraces have been 
referred to. The reader who may desire to follow this sub- 
ject farther will find assistance in the treatises named below.* 

General Distribution of Stream Terraces, — In North 
America stream terraces may be said to occur on the 
borders of nearly every river valley north of the central 
part of the United States, but are less conspicuous in the 
more south-eastern States and about the Gulf of Mexico: 
the reason being that the northern half of the continent 
was occupied by glaciers in late geological time, and has 
also undergone movements of the nature of elevation and 
depression throughout broad areas; while in the southern 
half of the continent there are no records of glaciation ex- 
cept on high mountains in the south-west, and evidences of 
recent changes of level are seldom pronounced. 

Stream terraces extend far southward from the formerly 
glaciated region, for the reason that the glaciers drained by 

^ I. C. Russell, " Topographic Changes Due to Landslides," Popular Science 
Monthly, vol. liii., i8g8, in press. 

^ G. K. Gilbert, U. S. Geoh^ical Survey, Monographs, vol. i., pp. 78-86, i8qo. 
W J McGee, CJ. S. Geological Survey, nth Annual Report, part i., pp. 256- 

273, 18S9-90. 



STREAM TERRACES 1 75 

southward-flowing streams furnished more debris than the 
streams could remove, and they became overloaded and con- 
sequently filled in their previously excavated valleys. When 
the glaciers disappeared and the streams were no longer over- 
loaded, they cut channels through their previously formed 
flood-plains, and left portions of them as terraces on their 
sides. This process was aided also by a general depression 
at the north, due to some extent, it is believed, to the 
weight of the ice, and in part to the effect of the lowering 
of the temperature to a considerable depth in the earth's 
crust and a consequent contraction and depression of the 
surface, and a partial re-elevation when the glaciers vanished. 

Many of the beautiful river-valleys of New England owe 
much of their attractiveness to the gracefully bending curves 
traced on their borders. Numerous towns and villages in 
that region are indebted for their sightly locations to the 
terraces on which they are built. Present flood-plains and 
abandoned portions of former flood-plains afford rich agricul- 
tural lands. These were among the first areas to be cleared 
and cultivated after European immigration began. A 
direct relation between the effects of distant and far-reach- 
ing changes in geography and the advance and growth of 
civilisation is here abundantly illustrated. 

What has been said of the terraced valleys of New Eng- 
land is true in varying degrees of a great area to the north 
embraced in the south-eastern provinces of Canada, and of a 
still broader region to the west including New York, Penn- 
sylvania, Ohio, and thence north-westward to the Pacific. 

The valleys of the Ohio and of many of its tributaries are 
noted for their terraces. In this region and also in the val- 



1/6 RIVERS OF NORTH AMERICA 

leys tributary to the upper Mississippi, the numerous terraces 
are due principally to the re-excavation of pre-glacial val- 
leys, in which overloaded glacial streams dropped the 
burdens too heavy for them to carry. The effects of the 
changes in drainage accompanying the great ice invasion at 
the north may be traced throughout the length of the Mis- 
sissippi, but become less and less conspicuous towards its 
mouth. 

The borders of the streams flowing to the Gulf of Mexico 
(other than the Mississippi), including the larger rivers of 
the Texas region and those draining the eastern slope of the 
Appalachians south of Maryland, are mostly without con- 
spicuous terraces. The streams in this region did not feel 
the direct influence of the glaciers at the north and have 
passed their period of youth, except on their extreme head- 
waters. The terraces they may have formed as a part of 
their normal aggrading and re-excavation have been re- 
moved principally by weathering, and the comparatively 
gentle slopes of their valleys show vertical scorings due to 
the action of rills and not the nearly horizontal lines which 
record higher water stages. Such terraces as do occur 
along the sides of southern rivers are in part remnants of 
ancient baselevel plains, or, in some instances, the result 
of local changes in elevation. These statements are inten- 
tionally made general, as it is only the more marked differ- 
ences between the characteristics of southern and northern 
valleys to which attention is here sought to be directed. 

The rivers flowing eastward from the Rocky Mountains, 
like the Platte, Missouri, Arkansas, etc., are bordered by 
terraced slopes in a portion of their valley tracts, and for 



STREAM TERRACES I 77 

many miles in the plains tracts after leaving the mountains, 
but well out on the Great Plains they are in part, and per- 
haps mostly, engaged at the present time in aggrading 
previously formed valleys, and terraces are not conspicuous. 

One reason for the presence of conspicuous terraces adja- 
cent to the mountains and in the valleys eroded in them 
previous to the great extension of the glaciers, is that the 
streams dropped the coarser and heavier portions of their 
loads at those localities, and bore on only fine material to 
their low-grade plains tracts. Subsequently, when the flood- 
plains thus formed were terraced, the escarpments in coarse 
and in part cemented gravel and boulders retained their 
slopes for a longer period than the similar escarpments in 
fine material a hundred miles or more downstream. In 
addition to this, it is to be noted that the gradients of the 
highest terraces are greater than the gradients of the streams 
as they flow at the present day. For this reason the verti- 
cal interval between the terraces and the present streams 
which they follow — that is, the height of the downward- 
sloping escarpments bordering them on their valley margins — 
progressively decreases from the gateways in the mountain 
to the sea. The lower terraces composed of soft material 
far out on the plains have melted down and been removed 
to a great extent, under the action of the atmosphere, while 
the higher terraces in firmer material have retained their 
characteristic topographic forms. 

In the terraces of the Rocky Mountains and adjacent 
portions of the Great Plains, the wide-reaching influence of 
the Glacial epoch is again recorded. The Rocky Mountains 
had their local glaciers at that time, and the streams were 



178 RIVERS OF NORTH AMERICA 

flooded especially during the final nielting of the ice, when^ 
it is inferred, the rain-fall was also abundant, but the swol- 
len streams were overloaded and their channels became 
deeply filled. The effects of more or less periodic changes 
in the elevation of broad areas above the sea may perhaps 
be made out in this region from the terrace records, but as 
yet too little attention has been given to this subject, and 
in fact to the general surface features of the Rocky Mountain 
region, to warrant one in offering anything more than pro- 
visional answers to the questions here suggested. 

In the Great Basin region variations of climate are again 
recorded by the work of the streams. Changes in the rate 
at which streams deposit, it will be remembered, depend on 
changes in velocity and in load. Velocity depends in part 
on volume. A change in climatic conditions from arid to 
humid means an increase in the volume of the draining 
streams. Whether this increment in velocity will be ac- 
companied by deeper corrasion or by aggrading, is deter- 
mined by the accompanying change in the rate at which 
the streams are loaded. An increased rain-fall would be 
accompanied by a greater removal of loose material from 
the highlands, and consequently a greater contribution of 
debris to the stream. There would, therefore, be a bal- 
ancing of conditions in any one part of a stream's course. 
If the supply of debris was not in excess of the transporting 
power of the draining streams, they would corrade, but if 
the loads delivered to them were too great, deposition 
would result. When the whole extent of a river is con- 
sidered, a change from arid to humid conditions must 
increase both corrasion and deposition. More active cor- 



STREAM TERRACES 1 79 

rasion in the upper portion of a drainage system usually 
necessitates a greater rate of deposition lower down. 

The streams of the Great Basin felt the effects of the 
change of climate which caused, or accompanied, the Gla- 
cial epoch, and, as the results indicate, were overloaded. 
Many of them are bordered by terraces in such a manner as 
to show that they were formerly of greater volume than at 
present, and subsequently decreased in volume, but were 
able for a time to cut channels and broaden their valleys in 
previously formed flood-plains. In many instances the 
streams have diminished to such an extent, however, that 
they are now aggrading. In their enfeebled condition they 
are unable to carry even the small burdens that are imposed 
upon them. 

In the region of the high plateaus drained by the Colo- 
rado and its branches, there are many terraces. The 
majority of those which attract the eye, however, are due 
to the weathering of the outcrop of alternating hard and 
soft strata, but stream terraces also occur. Many broad 
valleys are deeply filled and without terraces, as is illustrated 
by Fig. A, Plate XVI., owing to the fact that aggrading has 
been in progress throughout a large portion of the present 
arid period. 

The Colorado River, as is well known, flows through a 
canyon within a canyon. The outer canyon is in places 
some fifteen miles broad and comparatively level-floored. 
Sunken in this floor is the deeper, inner canyon, which in 
places is from one to two miles broad. The floor of the outer 
canyon is thus a great terrace, as may be seen on inspect- 
ing Plate XVII. The general history to be read in these 



l8o RIVERS OF NORTH AMERICA 

features is that the region was at one time lower than now 
by an amount about equal to the depth of the inner canyon. 
The Colorado cut down its channel to baselevel and then 
broadened it into a wide valley. Subsequent elevation 
renewed the energy of the stream and enabled it to cut 
down nearly to baselevel once more, leaving large portions 
of the bottom of its older valley as a terrace on each side of 
its later gorge. The terrace thus formed extends into 
many of the tributaries of the main river, and in fact the 
change to which it was due affected a large part if not the 
entire drainage system. 

Our knowledge of the terraces in the valleys of North 
America is too immature to permit us to state with confi- 
dence their distribution throughout the continent, and one 
more illustration must suffice for this brief review. 

Columbia River, in the portion of its course known as 
the Big Bend, flows through a narrow, canyon-like valley 
on the side of which there are numerous terraces. A short 
account of these taken from a report by the present writer,* 
will indicate not only the nature of the stream records so 
abundant on the sides of many of the canyons and valleys 
of the far Northwest, but also show how stream terraces 
are frequently associated with similar topographic forms 
originating in other ways: 

" On descending the side of the canyon [of the Columbia, op- 
posite the entrance of Chelan valley] by means of a road following 
a deep, high-grade gorge, we notice that there are many terraces 
on each side of the river. The most remarkable of these, and 

' T. C. Russell, "A Geological Reconnoissance in Central Washington," U. 
S. Geological Survey, Bulletin No. io8, pp. 78, 79, 1893. 



STREAM TERRACES l8l 

one of the finest examples of terrace structure that can be found 
anywhere, is a level-topped shelf formed of gravel and water- 
worn boulders, the surface of which is seven hundred feet above 
the Columbia. This truly remarkable terrace is best developed 
about two miles below where we descended into the canyon. It 
is there several hundred feet broad and runs back into lateral 
gorges, showing that the sides of the main canyon were deeply 
scored by lateral drainage before the gravel forming the terrace 
was deposited. On the west side of the valley there are other 
fragments of the same deposits, forming a less conspicuous shelf, 
which has been built against the steep slope, and has the same 
level as the great terrace on the east side of the river. The 
valley was excavated lower than the bottom over which the 
Columbia now flows, and then filled in from side to side with 
stream-borne stones, gravel, and sand before the present channel 
was excavated. In the re-excavation, fragments of the deposits 
filling the canyon have been left clinging to its slopes. Streams 
flowing down lateral gorges have cut channels across the terrace, 
thus revealing the structure even more plainly than the steep 
slope leading to the river. 

" Above the valley opening in the west wall of the canyon 
and leading to Lake Chelan, there are other large remnants 
of the same great terrace, this time on the west side of the 
river. In the broad plain formed by the surface of the terrace, 
there stands a lofty pyramid of solid rock completely surrounded 
by the gravel deposit and rising like an island from its level 
surface. 

" The terrace gravels extend into the valley of Lake Chelan 
and form conspicuous terraces about its lower end. For many 
miles both up and down the Columbia, other fragments of the 
same level-topped deposit occur, always forming striking features 
in the landscape, owing to the marked contrast of their smooth 
horizontal lines with the vertical line due to the erosion of rills 
and creeks. 

" Beside the great terrace described above there are many 
other but less conspicuous horizontal lines on the sides of the 
Columbia canyon. Some of these below the horizon of the main 



1 82 RIVERS OF NORTH AMERICA 

terrace are stream terraces, made by the river in lowering its bed. 
A more numerous but much less regular class are due to land- 
slides, of which there have been many. Other horizontal lines 
are due to the unequal weathering of the strata of basalt and of 
interstratified sedimentary beds. 

" Still another class of terraces, both numerous and conspicu- 
ous, has been formed as moraines on the sides of the glacier 
that once filled the canyon up to an elevation of 1200 feet above 
the river as it flows to-day. The moraine terraces are of older 
date than the great terrace described above, and about the en- 
trance of Chelan valley have been partially buried by it. 

'* In the canyon of the Columbia for several miles above Lake 
Chelan its rugged sides are strewn with thousands of perched 
boulders, left by the retreat of the ice. These have a definite 
upper limit, but mingled with them are masses of basalt that have 
fallen quite recently from the cliffs above. 

" In embayments along the sides of the main canyon and back 
of the ridges of stone and boulders left by the ancient glaciers, 
there are flat areas which have been filled in with fine material, 
washed from higher levels. These plains have in some instances 
been cut by small streams flowing across them, thus adding other 
horizontal lines to the complex topography of the canyon 
walls. 

'* It is not practicable to describe these terraces in detail, but 
those who visit Lake Chelan will have an opportunity to read for 
themselves the remarkable history which they record. In study- 
ing them, however, the traveller must bear in mind that the 
canyon, after being cut through various rocks to a depth greater 
than it now has, was occupied by a large glacier, and then by an 
arm of a large lake, and that river, glacial, and lacustral records 
are inscribed on the same slope. In addition, there have been 
many landslides, producing deceptive, terrace-like forms, and 
terraces due to the unequal weathering of hard and soft 
beds." 

A continuation of this review of the distribution of ter- 
races would lead us northward to the Mackenzie and 



STREAM TERRACES 1 83 

Yukon/ where many records similar to those just con- 
sidered are known to exist, but our present knowledge of 
the changes these streams have experienced is even more 
scanty than for the central and southern portions of the 
continent. 

The most important result of this hasty excursion through 
terraced river-valleys is perhaps the recognition of the fact 
that terraces exist along the sides of stream-cut valleys 
throughout the length and breadth of North America. 
Volumes of history are recorded in those graceful curves 
which give beauty to the varied scenery of valley borders 
from the tropical forests of Central America and Mexico to 
beyond the Arctic circle. The interpretation of these 
records has only recently been undertaken, and much that 
is new unquestionably awaits the patient explorer. The 
general principles to be used in this study have been pre- 
sented in the present chapter, but as investigation pro- 
gresses, much that is novel in details, and probably also the 
discovery of as yet unknown laws or modifications of those 
now recognised, will reward the student. 

* I. C. Russell, •' Notes on the Surface Geology of Alaska," in Bulletin of 
the Geological Society of America, vol. i., pp. 144-146, 1890. 



CHAPTER VII 

STREAM DEVELOPMENT 

Consequent Streams, — In case a portion of the sea floor 
should be upraised so as to make what geographers term a 
new land-area, the streams flowing from it would take the 
easiest courses, as determmed by the slope of the surface,, 
regardless of the structure of the rocks beneath. If a 
dome-shaped uplift should occur in a broad plain underlain 
by previously horizontal layers of rock, the rain falling on 
its surface would form rills, and these uniting would give 
origin to creeks and perhaps to rivers, which would flow 
away in all directions from the summit portion of the uplift. 
In each of these hypothetical cases the streams would evi- 
dently have their directions determined by the pre-existing 
topography, and hence may be termed conseqiieiit streams. 

Subsequent Streams, — As consequent streams deepened 
their channels they might discover differences in the rate 
at which they can remove the rocks, and hence have their 
directions variously modified by differences in hardness or 
by the greater or less solubility of the various beds that 
they encountered. New branches or tributaries to the main 
streams would be developed, the positions of which would 
be determined by the down-cutting of the channels of the 

184 



STREAM DEVELOPMENT 1 8 5 

master streams, and by the relative ease with which the 
various rocks coming to the surface could be removed. 
That is, as a drainage system develops, streams originate^ 
the directions of which are regulated by the hardness and 
solubility of the rocks. Such streams appear subsequently 
to the main topographic features in their environment, and 
are termed subsequent streams. 

Ideal Illustration of Stream Adjustment and Development. 
— Perhaps the best way in which to obtain a graphic idea 
of the changes passed through by a river system in the 
course of its development and adjustment to the geological 
conditions it discovers, when unaffected by marked climatic 
changes and not seriously disturbed by movements in the 
earth's crust, is to form a mental picture of a gently sloping 
plateau with an essentially even surface on which rain falls 
and gives origin to rills and brooks which unite to form 
larger consequent streams, and picture to ourselves the 
changes that would result under the influence of a moder- 
ately humid climate. Imagine such a plateau, we will say, 
one hundred miles long from right to left and fifty miles 
broad, sloping gently toward an assumed point of view, and 
follow in fancy the changes in the streams, and the result- 
ing modifications in the relief of the surface as normal 
stream development progresses. To complete our concep- 
tion, we may assume that, beyond the sky-line bounding the 
far side of the landscape, the surface gently declines in 
the opposite direction. That is, the plateau before us is 
the side of a long ridge, the central axis of which is raised, 
we will say, one thousand feet above sea-level. 

The rocks beneath the surface of the lilted plane, we will 



I 86 RIVERS OF NORTH AMERICA 

assume, are in inclined layers, which slope toward our point 
of view, at a greater angle than the surface of the plane, and 
besides are composed of hard and soft beds. A section 
through the tilted plane at right angles to the axis of the 
uplift has the structure indicated in the following diagram. 




Fig. 15. Section at Right Angles to the Lines A A and B B in Fig. 16. 

The harder rocks are shaded. The lines in which the inclined 
beds join the surface of the plane run in the direction of its 
length. The conditions here assumed are such as might 
result if a region underlain by inclined stratified rocks had 
been planed away nearly to baselevel and then upraised so 
as to produce a gently sloping peneplain. 

The rain-water, falling on the surface of the plane before 
us, gathers in part in depressions on its surface and forms 
ponds and lakes. These are but temporary, however, as 
the shallow basins are soon filled with sediment, or have 
their outlets cut down by the overflowing water, and are 
drained. The rills supplied directly from the rain and those 
starting from the lakelets unite to form larger streams, which 
flow down the inclined plane to the sea in obedience to 
gravity. The directions of these initial streams are deter- 
mined by the slope of the surface. They are, therefore, 
consequent streams. Their number will be determined by 
the inequalities of the surface which cause the rills and 
rivulets to unite. Some will be longer than others. Some 
will have a greater volume than others. A common feature, 
shared at first by all, is that they have but few tributaries. 



STREAM DEVELOPMENT 



187 



A diagram showing these initial consequent streams, in 
their infancy, is presented in the following ideal map/ 




Fig. 16. Ideal Sketch-Map Showing Young Consequent Streams. 

The consequent streams, a to i, follow courses determined 

by the slope of the surface and approximately straight. The 

hardness or softness of the underlying rock does not affect 

them at first, for the reason that they are surface streams. 

They differ in volume, as is indicated to some extent by the 

length of the lines representing them in the diagram. All 

^ Figures 16, 17, and 18, together with almost all of the account of the 
development of streams here presented, have been taken from a highly instruc- 
tive article by W. M. Davis, on " The Development of Certain English Rivers," 
in The Geographical Journal {oi the Royal Geographical Society), vol. v., pp. 
127-146. London, 1895. 



I 88 RIVERS OF NORTH AMERICA 

of them begin at once the task of deepening their channels 
to a certain grade determined by their volumes and their 
loads. This work would not progress at the same rate in 
all the streams because they are of unequal length, of differ- 
ent volumes, and are variously loaded. The longer streams 
would, under most conditions, be the larger, and would 
corrade their channels most rapidly. The drainage from 
the inter-stream spaces flows to the master streams and de- 
velops feeding branches. The gradients of these branches 
depend on the rate at which the master streams deepen their 
channels. The branches of the more rapidly corrading 
master streams have their velocities, and consequently 
their corrading power, increased more rapidly than the 
similar branches of their weaker neighbours. Hence the 
branches of the stronger consequent streams cut back by 
head-water corrasion and increase in length more rapidly 
than the branches of the weaker consequent streams. More 
and more of the water falling on the territory between the 
main streams is thus carried to the more favoured conse- 
quent streams, and increases still more their advantage 
over their weaker neighbours. 

The initial streams, it will be remembered, flowed down 
the original slope, as shown in Fig. 15, at right angles to 
the strike, that is, across the edges of the strata composing 
the tilted block of the earth crust we have in mind ; the 
branches of the streams, however, flow parallel with the edges 
of the strata where they come to the surface, and hence 
find hard and soft bands parallel with their courses. As 
erosion progresses, the edges of the resistant beds are left 
in relief, forming ridges, while the less resistant beds are 



STREAM DEVELOPMENT 



189 



more rapidly removed and determine the courses of the 
subsequent branches. As this process goes on, our sloping 
plane loses its smoothness, channels are cut by the conse- 
quent streams, and depressions are made by their branches 
trending in general at right angles to their courses. Be- 
tween the lateral valleys ridges appear, marking the posi- 




FiG. 17. Ideal Sketch-Map Illustrating Stream Development. 

tions of the edges of the resistant beds. These ridges are 
at first straight, and mark the intersection of the hard beds 
with the original, gently sloping surface. As the strata are 
inclined, however, one side of a ridge formed by the out- 
cropping edge of a hard bed will have a more gentle slope 
than the opposite side. The sides of the ridges slope gently 



IQO 



RIVERS OF NORTH AMERICA 



toward our assumed point of view and present steep escarp- 
ments in the opposite direction. The condition of the 
sloping surface before us at this stage is shown in Fig. 17. 
The hard layers, indicated by broken bands, stand up as 
ridges, and the branches of the original consequent stream 
have begun to develop valleys in the soft rocks. 




Fig. 18. Ideal Sketch-Map Showing an Advanced Stage in Stream Develop- 
ment. Former Shore Shown by Broken Line 

The consequent stream c, being stronger and corrading 
more rapidly than a, deepens its channel through the hard 
ridge A A more rapidly than its weaker neighbour. The 
branch m' of the strong consequent stream c, having its 
place of discharge lowered by the corrasion of the stream 



STREAM DEVELOPMENT I9I 

to which it is tributary, is able to remove the soft rocks 
forming its bed and to grow in length by head-water corra- 
sion more rapidly than the corresponding subsequent branch 
of a. As in' increases in length, it captures more and more 
of the drainage of a, thus weakening that stream, and at 
length draws off all of its water above 0. The original 
stream a is thus broken in two, or is beheadedy to use a 
term proposed by Davis. The notch that a has cut in the 
ridge of hard rock A A is left as a wind-gap. The bottom 
of this gap is a divide from which the waters flow each way. 
The beheaded stream a' holds its former course below the 
divide, but is weakened by the loss of its head-waters; 
the portion of the stream a, from the divide on the hard 
bed to where the subsequent stream m' intersected it, is 
reversed. For such reversed streams Davis has proposed the 
name obseqitent. As time goes on, changes similar to those 
accompanying the backward cutting of in' take place also in 
the other streams, as may be seen from Fig. 18, in which 
a more advanced stage in stream development is indicated. 
Another feature of the changes in progress is shown by 
the fact that the ridges of hard rock, indicated by the 
bands A A and B B, are no longer straight. When the 
stronger streams cut through them, forming water-gaps, 
the cliffs recede by weathering and by the sapping of their 
bases by the streams. This wasting of the hard ridges goes 
on most rapidly on the side toward which they slope most 
steeply; that is, on the farther side from our assumed point 
of view. The cliffs, formed by the steeper slope of the 
ridges of hard rock, thus gradually migrate in the direction 
of the dip of the hard beds, that is, toward our point of 



192 RIVERS OF NORTH AMERICA 

view, under the influence of general erosion and sapping 
throughout their entire length. But this recession is most 
rapid where the stronger consequent streams cross them, 
and they become lobed or scalloped, as shown in Fig. 17. 
With the process of stream development, the alignment of 
the cliffs becomes more and more modified, until the reces- 
sion in the neighbourhood of the master stream is checked 
by the streams having cut down nearly to baselevel. At 
this stage, the cliffs at the ends of the V-shaped gorges, at 
the apex of which the master streams cross the hard beds, 
will remain stationary and crumble away, while those por- 
tions of the cliffs between the master streams, which before 
receded more slowly than the portions near the stream, will 
retreat more rapidly than the portions of the escarpment 
near where the master streams have cut to baselevel. In 
an advanced stage of stream adjustment and of topographic 
development, the lines of cliffs, at first straight and then 
deeply lobed, will again approach an even alignment, but 
the position of the ridges will change with this development, 
and move in the direction of the dip of the hard beds. The 
-cliffs in an early stage of development were of faint relief; 
when cut into deep lobes, they stand up prominently, but 
as their alignment is again established, they become sub- 
dued, and when the process is far advanced nearly or quite 
disappear. 

To return to the development of drainage. The manner 
in which the subsequent stream m\ Fig. 17, captured and 
diverted the head-water of a and divided that stream into a 
beheaded portion, a\ a reversed portion, 0, and a diverted 
portion, a" ^ will serve to illustrate the similar process fol- 



STREAM DEVELOPMENT I93 

lowed by other streams. In each instance, the subsequent 
branches of the stronger primary stream cut back until they 
divert a portion of the drainage of their weaker neighbours. 
The result of this process can be easily predicted from a 
study of the map. At a certain stage in the process, the 
stronger streams c and h, will have captured the head-water 
of all of their rivals above the ridge A A, and a competition 
between the two conquering streams will ensue. An ad- 
vanced stage in this struggle is indicated in Fig. 18, where 
the subsequent branches m!' and n" of the stronger mas- 
ter stream c have captured and diverted the head-waters 
of >^^ 

In the map forming Fig. 18, it will be noted that the 
ridges of hard rock are again nearly straight, and also that 
the lower courses of c and h have become tortuous. The 
reason for the curves in the lower courses of the stronger 
stream is that after cutting down their channels nearly to 
baselevel and becoming sluggish, they continue to corrade 
laterally and form flood-plains on which they meander from 
side to side, at the same time broadening their valleys. 
As has been shown on a previous page, a sluggish stream, 
having little power to overcome obstacles, is more easily 
deflected than a swifter stream, and besides, the lower por- 
tions of the valley and plains tracts of a stream during ad- 
vanced stages in development are regions of deposition. 
The migrations of the streams a and c have brought them 
together, as shown in Fig. 18, thus illustrating another 
process of capture. 

The examples of stream development we have been fol- 
lowing are ideal, but, I believe, true to nature. The reason 



194 RIVERS OF NORTH AMERICA 

for sketching an ideal illustration is that in nature various 
disturbing conditions usually modify the process and in- 
crease the difificulty of separating what is normal from that 
which may be termed accidental. Stream development is 
a slow process even when not disturbed by marked climatic 
changes or by movements in the earth's crust. The life of 
a man, or even of a nation, is too short to embrace the time 
necessary for the development of a river system. It is only 
by studying many streams in various stages of their develop- 
ment that geographers are able to sketch generalised pictures 
of the normal changes a great river passes through in its life- 
span of millions of years. 

When one attempts to apply the elementary conceptions 
of stream development outlined above to actual streams, it 
is found that many modifying conditions have to be taken 
into account. Broad surfaces with even initial slopes are 
rare; the rocks forming the earth's crust are frequently 
folded and faulted, especially in uplifted regions, and con- 
sequently the development of subsequent streams is fre- 
quently greatly modified; more puzzling complications 
arise, however, from the fact that the land is not stable, 
but is subject to up-and-dov/n movements, which disturb or 
entirely arrest the slow process of stream development be- 
fore it can run its normal course. These and still other 
modifying conditions have to be considered in studying the 
history of the streams which drain the land and throughout 
their history are continually modifying the relief of the 
surface. 

With the introduction to the principle of stream develop- 
ment in mind, let us turn to a portion of our own land 



STREAM DEVELOPMENT 1 95 

where the processes just outlined have been long in action 
and see if we can read a portion of the history recorded by 
the valley and intervening hills. 

EXAMPLES OF STREAM DEVELOPMENT AND ADJUSTMENT 
IN THE APPALACHIAN MOUNTAINS 

The leading geographical feature of the Appalachian 
Mountains, more especially of their northern half, is the 
large number of curving but generally parallel, level-topped 
ridges with valleys between, which compose the uplifted 
region. Crossing the ridges and valleys approximately at 
right angles is a series of rivers, such as the Delaware, Sus- 
quehanna, Potomac, and James, which have their sources to 
the west of the mountains, and flow eastward to the sea. 
These master streams receive many branches from the valleys 
crossed by them. The general features referred to are 
shown on the map forming Plate IX. 

The ridges in the northern Appalachians are known to 
be due to folds in the rocks, which have been truncated or 
planed off to a certain general level, and the surface thus 
formed upraised and eroded so as to leave the edges of hard 
beds in bold relief. The first question suggested by an in- 
spection of the accompanying map is : How has it come about 
that the main rivers flow through the ridges of hard rock by 
means of gaps cut in them, instead of being turned aside 
and pursuing what would seem to be much easier courses to 
the sea? The ideal case of river development we now have 
in mind will assist in solving this problem. 

The study of the northern Appalachians, conducted by 
a large number of geologists, and especially by Davis and 



196 RIVERS OF NORTH AMERICA 

Willis, has shown that after the rocks were folded the region 
existed as a land area, probably more elevated than now, 
and was worn down nearly to sea-level, or, in other words, 
was reduced to the condition of a peneplain. This pene- 
plain was subsequently elevated and tilted so as to slope 
toward the south-east. The conditions were then essentially 
the same as in the ideal case already discussed, except that 
the rocks beneath the tilted plane had a complex structure. 
The rocks were also of many degrees of hardness and solu- 
bility. The south-eastern margin of this tilted peneplain 
was at sea-level, while its north-western border, in the region 
now embraced in the central and western portion of New 
York, Western Pennsylvania, and West Virginia, was ele- 
vated, not all at once, but slowly, to a height of probably 
two or three thousand feet. 

We have designated the sloping surface referred to as a 
tilted plane, more accurately it should be considered as the 
side of an elongated dome-like uplift. 

The pre-existing streams flowing with slack currents in 
their old age, and young consequent streams originating on 
the tilted peneplain, took the direction of easiest descent, 
and flowed south-eastward to the sea. The courses of these 
streams were determined by the slope of the surface irre- 
spective of the position or character of the rocks beneath, 
and hence are consequent streams. As they deepened their 
channels, the edges of hard and soft beds were cut through. 
With this process of deepening the channels of the conse- 
quent streams, many subsequent branches originated which 
also entrenched themselves, but their directions were con- 
trolled by the hardness or solubility of the rocks. Those 






., I 




*v 



LW"V> 







1-: f^--^^^:#\ 






The Nortliern Apj^alachiaii 
Apf^roxiinate scale: one in 











After liailey Willis.) 
>venty-seven miles. 



STREAM DEVELOPMENT I97 

originating on soft beds maintained their positions, while 
those which flowed at first along the outcrops of hard beds 
were soon shifted to the softer beds adjacent. The hard 
beds were thus left as ridges between the valleys excavated 
by the subsequent streams along the outcrops of soft beds. 
As the master streams flowing south-eastward lowered their 
channels their subsequent branches were given greater 
velocity and deepened their channels also. This sinking 
of the rivers and of all their branches produced a roughening 
of the topography and the once nearly smooth plain became 
a rugged mountainous region. In this general down-cutting 
the large consequent streams first reached baselevel at their 
mouths, and then a low gradient, or an approximation to 
baselevel, was produced progressively up stream. The tend- 
ency of all the streams, or their chief aim, as we may say, 
was to reduce the land to a second baselevel. This would be 
accomplished by the downward corrasion of the streams in 
their upper courses, and a broadening of their valleys in 
their lower courses ; the broadening process progressing up 
stream as fast as the valleys were deepened at a certain 
progressively decreasing rate. 

During the process outlined above, each of the subsequent 
branches of the main streams entered into competition with 
its neighbours for the possession of the territory between 
them, as in the ideal illustration of stream development 
previously considered. The branches of the larger master- 
streams, by having their places of discharge lowered more 
rapidly than adjacent subsequent streams flowing to weaker 
consequent streams, were able to extend their head branches 
and capture new territory in the manner already discussed. 



198 



RIVERS OF NORTH AMERICA 



This process was modified in many ways, however, owing 
to complex folding in the rocks exposed by erosion. 

Influence of Folds i?i the Rocks on Stream Adjiistvient, — A 
fold in stratified rocks of various degrees of resistance, when 
the axis is horizontal, will produce parallel ridges and valleys 
with tapering ends. If the axis of the fold is not horizon- 
tal but inclined so as to pass below a horizontal plane in one 
direction and rise above it in the opposite direction, it is 
evident that if the region where the fold occurs is carved 
away to a horizontal plane, and then etched so as to leave 
the edges of the hard layers in relief, the resulting ridge 
will not be parallel throughout, but form more or less 
elliptical curves/ 




Synclinal Fold, with Central 
Canoe-Shaped Valley. 



Anticlinal Fold, with Hemi- 
Cigar-Shaped Mountain. 

Fig. 19. Topographic Forms Resulting from the Erosion of Folded 
Rocks. (After Bailey Willis). 



The topographic changes resulting from the weathering 
and erosion of rocks of various degrees of resistance, when 

' Folds in the rocks, if traced to where they die out, will be found either to 
flatten and spread so as to merge with undisturbed areas or become narrow and 
more or less sharp-pointed. Individual folds are more or less conical and when 
cut by planes of erosion give figures which are conic sections. 



STREAM DEVELOPMENT 1 99 

folded, are shown in Fig. 19. In one instance the fold is 
downward, so that the strata on the borders dip toward the 
longer axis, and is termed a synclinal ; and in the other 
instances the strata forming the arch dip away from the 
longer axis, making an anticlinaL 

Water-Gaps and Wind-Gaps, — The Appalachian Mount- 
ains are due to the upraising of a great belt of country in 
which the rocks have been folded into anticlinal and syn- 
clinal, as in the illustration just given. The longer axes of 
the folds trend N. E. and S. W., and are either horizontal 
or pitch at various angles. The western side of each fold 
is usually steeper than the eastern side. The ends of the 
folds frequently overlap, one dying out and another begin- 
ning and continuing sometimes for scores of miles, before 
it in turn disappears and is replaced by another similar fold. 
It is the ridges in these variously truncated folds, due to 
the weathering out of resistant la3^ers, that give to the 
Appalachians their highly characteristic topography. The 
softer beds have been eroded away by the subsequent 
streams. The ridges of resistant rock form the divides be- 
tween the branches of the large rivers. The crests of the 
ridges are nearly level for the reason that the region was 
worn down to a peneplain before the etching process which 
gave them prominence was initiated. These level crest- 
lines are broken, however, by deep notches where the 
master streams pass through them, and are also indented 
by less deep notches where streams which have been 
beheaded formerly crossed them. 

The process of river conquest, as it has been termed, by 
which notches have been left in the crest-lines of the ridges. 



200 



RIVERS OF NORTH AMERICA 



is illustrated by the following typical example in Virginia, 
borrowed from an admirable essay on the northern Appa- 
lachians, by Willis.' 

The Potomac near Harper's Ferry flows through two deep 



THE ^ 
KITTATINNV y^ /^ 
PLAIM yfC .y 








-.-'^■^*^^^»x-,^^4oV /> 




( 5V <</ 






^reeff 








Arrangement of Streams on the 
Kittatinny Peneplain. 



Adjusted Streams on the 
Shenandoah Peneplain. 



Fig. 20. Methods and Results of River Piracy. (After Bailey Willis.) 

picturesque notches in ridges of hard rock, as may be seen 
from the photograph forming Fig. B, Plate XVL Such a 
notch in a mountain crossing the course of a stream, and still 
occupied by the stream which excavated it, is known in the 
language of geography as a water-gap. In the Blue Ridge, 
a few miles south of Harper's Ferry, there is a similar 
notch, but not so deep, the bottom of which, like the crest 

' Bailey Willis, " The Northern Appalachians," in National Geographic Mono- 
graphs, vol. i., pp. i6g-202, published by the American Book Co. under the 
auspices of the National Geographic Society. 



STREAM DEVELOPMENT 201 

of the ridge to the north and south, is a divide between the 
streams flowing east and those flowing west. The air-cur- 
rents flow through such notches, and this has gained for them 
the name of wind-gap among the inhabitants of the Appa- 
lachian region. This familiar name has been adopted by 
geographers as a generic term by which to designate a class 
of notches in the crest-lines of ridges and mountains having 
a certain origin. The particular notch in the Blue Ridge 
here referred to is known as ** Snickers Gap/' and is a typi- 
cal illustration of a wind-gap. Its history is shown graphi- 
cally on the two accompanying sketch-maps by Willis. 

It will be remembered that the country about Harper's 
Ferry was at one time in the condition of a tilted peneplain, 
and that a strong consequent stream, the Potomac, flowed 
across it toward the east ; and that this river and its tribu- 
taries deepened their channels and entrenched themselves in 
the plain. As a result of this process and of general erosion, 
the edges of the layers of hard rocks beneath the surface 
of the plain were left in relief. The Blue Ridge, composed 
of hard, resistant quartzite, is a typical illustration of a ridge 
originating in this manner. 

The conditions after the Potomac had begun to deepen 
its channel, and the Blue Ridge was a line of faint relief, 
are shown in the ideal sketch-map to the left of Fig. 20. 
The infant Shenandoah River entered the Potomac above 
the water-gap at Harper's Ferry, flowing northward along 
the west base of the Blue Ridge in a valley excavated in 
limestone. At the stage in the history shown in the map 
just referred to, the Shenandoah was a young subsequent 
stream. To the south of the Potomac, as indicated on the 



202 RIVERS OF NORTH AMERICA 

map, there was another but weaker consequent stream, 
Beaverdam Creek, which also crossed the Blue Ridge in a 
water-gap. It will be remembered that all of these streams 
were flowing at a higher level than the Shenandoah occupies 
to-day, and that the Blue Ridge was a much less prominent 
topographic feature than at present. The Potomac, being a 
larger stream than . Beaverdam Creek, deepened its channel 
more rapidly, and thus lowered the mouth of the Shenandoah 
and caused it to flow more swiftly. The Shenandoah was 
thus enabled to deepen its channel and to extend its head 
branches more rapidly than its neighbour and rival. As a 
result, the Shenandoah captured the drainage of Beaverdam 
Creek to the west of the Blue Ridge, thus beheading that 
stream. The ability of Beaverdam Creek to deepen its 
channel in the hard rocks forming the Blue Ridge was thus 
lessened, and as the Shenandoah continued to lower its chan- 
nel, the portion of Beaverdam Creek situated between the 
places of capture and the bottom of the notch it had cut in 
the Blue Ridge was reversed, and also contributed its waters 
to the capturing stream. Beaverdam Creek was thus broken 
in two at the place where it formerly crossed the hard layer 
forming the Blue Ridge, and stream corrasion there ceased. 
The notch previously made was thus changed from a water- 
gap to a wind-gap. Subsequently the Blue Ridge became 
more and more prominent owing to the removal of the softer 
rocks on each side of it, but the notch in its crest-line re- 
mained. The conditions as they exist at the present day, 
when the bottom of the notch is a water-parting or divide, 
between streams flowing in opposite directions, is shown on 
the right-hand map of Fig. 20. Another illustration of the 



STREAM DEVELOPMENT 20$ 

robbing of one stream by a more favourably circumstanced 
rival is illustrated in Plate XIII, and described in advance 
in connection with a discussion of the migration of divides. 

Stream Conquest, — The process of capture by the subse- 
quent branches of strong consequent streams, just illustrated, 
has gone on, with various modifications in detail, through- 
out the Appalachians, and great variety both in the direction 
of the streams and in the relief of the land has resulted. 
The principal conditions which give one stream an advantage 
over its neighbour on the opposite side of a common divide, 
are a shorter course to the sea, greater volume, and softer 
rocks in which to sink its channel. 

If one of two streams heading against a common divide 
has a shorter course to the sea than its rival, other condi- 
tions being the same, its gradient will be greater, and hence 
it will have greater velocity and be able to corrade more 
rapidly, and, also, the amount of rock to be removed in 
order to reach baselevel is less. If the rain-fall on one side 
of a mountain range is greater than on the opposite side, 
other conditions being the same, the streams on the side 
having the greater rain-fall will be larger, and hence able to 
deepen their channels and extend their head branches more 
rapidly than the streams on the opposite side of the divide. 

In a similar way, it will readily be seen that of two com- 
peting streams, one flowing over hard and the other over 
soft rocks, the one with the easier task will deepen its chan- 
nel more rapidly than the one flowing over hard rock, and 
lead to the capture of some of the territory previously 
draining to its rival. 

In the sculpturing of the Appalachian Mountains all of 



204 RIVERS OF NORTH AMERICA 

these variations in conditions, and possibly still others, have 
been in progress at the same time. Many of the features 
of the northern Appalachians resulting from this process of 
the adjustment of streams to the structure of the rocks into 
which they sink their channels may be read on the map 
forming Plate X. 

This map illustrates in an admirable manner the way in 
which the Susquehanna, a strong consequent stream, flows 
across the edges of both hard and soft strata, independently 
of the topography, while the secondary streams follow the 
outcrops of the soft rocks, although occasionally flowing 
directly through a ridge of hard rock, and continuing their 
general course in the next valley. Evidently the secondary 
streams, flowing through ridges of hard rock, had their right 
of way established on the original peneplain, or, by a process 
of adjustment, described below, have made for themselves a 
way out of synclinal valleys and into depressions excavated 
in the tops of anticlinal ridges. 

The student should study also the well-developed den- 
dritic drainage in Lebanon valley at the bottom of the map, 
where the rocks are soft limestone which yield readily to 
solution ; in comparison with the remarkably straight course 
of Stony Creek, confined between two ridges of hard sand- 
stone. The crooked courses of the streams, particularly 
where they have cut down their channel nearly to the level 
of the master rivers, indicate the tendency of streams well 
advanced in their task of valley-making, to become sluggish 
and meander. Especially does this excellent map illustrate 
the fact that the relief of the land is due to the action of the 
stream in removing soft rocks and allowing the hard rocks 



Plate X. 



r 

I . 



"1 i': 



l^ • :^^^<y-^ '^l^^ 




Western Portion of the Anthracite Basin, Pennsylvania, Showing Canoe-Valleys 
and Mountains and the Course of the Susquehanna across them. (After 
Bailey Willis.) 

Approximate scale : one inch = twelve miles. 



STREAM DEVELOPMENT 20$ 

to stand in relief. Instead of the topography controlling 
the stream, the stream gives origin to the topography. 
As long since explained by Hutton, each stream flows in a 
valley of its own making. 

Ancient Peneplains. — Only a part of the history of the 
northern Appalachians has been read when the adjustment 
of the streams to geological structure and the growth of 
certain drainage areas at the expense of others has been 
worked out. The land has not remained stationary during 
the millions of years required for this process, but have been 
upheaved and depressed. One immensely long period of 
rest is recorded by the broad, featureless peneplain which 
was tilted, as already explained, so as to cause the master 
stream to flow across the future site of the mountains to the 
sea. A portion of this peneplain not yet consumed forms 
the Kittatinny Mountains in Eastern Pennsylvania. The 
plain referred to is hence named the Kittatinny peneplain.' 
After this plain had been deeply dissected and the streams 
had broadened their valleys so that only isolated remnants 
of the rocks remained above sea-level, the region was again 
elevated, but the second upward movement was not so great 
as the one just considered, and another attempt to reduce 
the region to baselevel was begun. A second peneplain was 
formed, but not carried to completion before the region was 
again raised. As stated by Willis, the upheaval leading to the 
dissection of the Kittatinny peneplain, caused an elevation of 
two hundred feet in New Jersey, six hundred feet in Pennsyl- 

^ Also known as the " Schooley peneplain" in New Jersey. See " Physical 
Geography of New Jersey," in vol. iv., of the Final Report of the State 
Geologist of New Jersey, p. 85, i8g8. 



206 RIVERS OF NORTH AMERICA 

vania, one thousand seven hundred feet in Southern Virginia, 
and thence southward decreased to the Gulf of Mexico. A 
typical illustration of this second peneplain is found in the 
broad bottom of the Shenandoah valley, and for this reason 
it has been named the Shenandoah peneplain. In the illus- 
tration forming Fig. B, Plate XVI, the Shenandoah pene- 
plain is on a level with the point of view, while the hills 
rise to the level of the Kittatinny peneplain. 

In consequence of renewed elevation after the broadening 
of the Shenandoah peneplain was well under way, the 
energy of the streams v/as again revived. Once more fall- 
ing swiftly, they have sawed and are sawing their channels 
down, and are preparing for the development of a future 
baselevel. 

In the southern Appalachians, the Kittatinny peneplain 
was not so completely developed as farther north, and a 
group of mountains representing the previous uplands from 
which the plain was carved was left. These mountains 
have maintained their existence to the present day, and still 
furnish the highest and most picturesque peaks in the sys- 
tem. This great group of peaks, of which Mount Mitchell, 
Rhone Mountain, and other prominent summits in Eastern 
Tennessee and Western North Carolina are examples, it will 
be seen, are of the nature of remnants rising above a broad 
and nearly completed peneplain, or, to use a technical name 
previously explained, they are monadnocks. 

At the south, the great Kittatinny peneplain was not tilted 
south-eastward as in Eastern Pennsylvania and adjacent 
States, but toward the south-west. Hence the streams 
flowing: down its inclined surface had their sources to the 



STREAM DEVELOPMENT 20/ 

east of the ridges and mountains subsequently developed 
by the erosion of its surface and flowed westward to the 
Mississippi and south-westward to the Gulf of Mexico. The 
principal consequent streams in this region are New River 
and the Tennessee. The Coosa at present belongs also 
in this category, but, as has been shown by Hayes/ was 
formerly a continuation of the Tennessee. 

Synclinal Mountains and Anticlinal Valleys. — The process 
of stream adjustment to geological conditions, discovered as 
erosion progressed, was much the same in the southern as 
in the northern Appalachians, but the details were in some 
respects different. One of the characteristic features in the 
structure of the southern Appalachians, but more especially 
of their western half, which departs somewhat widely from 
the typical structure at the north, is the presence of broad 
downward curves in the rocks, or synclinals, separated by 
comparatively narrow upward folds, or anticlinals. If the 
land had been elevated without being eroded, we should 
find to-day a series of prominent but narrow ridges run- 
ning north-east and southward, intervening between much 
broader, trough-shaped valleys. If streams should come 
into existence on such a surface where it was inclined in 
the direction of the longer axis of the ridges and troughs, 
they would evidently follow the depressions as consequent 
streams. 

In marked contrast to what would have been the topo- 
graphy of much of Eastern Tennessee and the northern 
portions of Alabama and Georgia had there been no ero- 

' C. Willard Hayes, " Geomorphology of the wSouthern Appalachians," in 
The National Geographic Magazine, vol. vi., pp. 109-iig, 1894. 



208 RIVERS OF NORTH AMERICA 

sion, we find that where the ridges would have been, there 
are now valleys occupied by well-developed drainage sys- 
tems, while the synclinals, which would have been valleys 
under the conditions just assumed, in reality stand in relief 
and form broad ridges or mountains, with shallow depres- 
sions in their surfaces. This reversion of what would have 
been the topographic relations of the anticlinals and syncli- 
nals had there been no erosion, is one of the most interest- 
ing chapters in Appalachian history. Let us see how this 
topographic revolution has come about. 

A good example of a synclinal plateau is furnished by 
Lookout Mountain, which terminates at the north in a bold 
escapement over one thousand five hundred feet high at 
Chattanooga, Tennessee, and extends south-west about 
seventy miles to Atalla and Gadsden in Alabama. It was 
at the extreme northern end of this synclinal, the axis of 
which declines gently southward, left in bold relief by the 
erosion of the bordering valleys, that the battle of Lookout 
Mountain was fought. To the west of Lookout Mountain 
is a deep anticlinal valley from four to five miles wide, and 
to the west of this, again, another broad synclinal plateau 
known as Sand Mountain. The present relief and drainage 
of this region are shown on the sketch-map forming Fig. 4, 
Plate XL 

The origin of the present strongly pronounced and char- 
acteristic topography of the region just referred to, has been 
studied by Hayes, and I cannot serve the reader better 
than by presenting an extract from his report.* 

^ Geological Survey of Alabama^ Biilletiu, No. 4, pp. 23-29, and Plate I 
1892. 



STREAM DEVELOPMENT 209 

The four maps presented on Plate XI. illustrate four suc- 
cessive stages in the history of the streams in the Lookout- 
Sand mountain region. An early stage in the topographic 
development is shown in Fig. i, where a stream flowed 
southward in each of the synclinal troughs and received 
tributaries from the adjacent anticlinal ridges. 

*' At first these streams were all flowing upon the same kind of 
rock, probably coarse sandstone, so that they were able to erode 
their channels most rapidly where the fall and consequently the 
transporting power of the stream was the greatest. But the slope 
of the synclinal troughs was very slight, so that the main streams 
had little power to deepen their channels, while the side streams 
flowing into them from the intervening ridge, although they were 
much smaller, still by reason of their greater fall eroded their 
beds more rapidly. 

** If the rocks of this region had been uniform in character for 
a long distance down from the surface, the effect of the more 
rapid cutting of the side streams would have been simply to re- 
duce the height of the intervening ridge, leaving the main streams 
in their original position in the synclinal troughs. But the rocks 
are not homogeneous. They consist of alternating hard and soft 
beds, and after the side streams had cut down a few hundred feet 
they came to layers of shale and then limestone which they could 
remove much more rapidly than the overlying sandstone. As 
soon as a side stream reached these soft rocks at any point, it 
tended to widen its valley at that point by removing the soft rocks 
so as to undermine and thus break down the overlying harder 
beds. By a continuation of this process lateral valleys were 
formed. Fig. 2 [Plate XI], extending in the direction of the ridge 
and at right angles to the side streams. 

** The two streams w and w' cut away at the divide d and 
the stream w having the lowest outlet was able to erode more 
rapidly than w and so pushed the divide farther and farther 
toward the side stream /' till finally it tapped the latter and led 



EXPLANATION OF PLATE XL 

Sketch map of a portion of Lookout Mountain, Wills Valley, and Sand 
Mountain ; showing various stages in the development of the present drainage 
system and topography. 

Fig. I. Showing the undulating surface which determined the initial posi- 
tion of the streams. B, B and T, T flow in synclinal troughs and receive side 
streams 1, 1, 1 and s, s, s from the intervening anticlinal ridge. 

Fig. 2. Showing a stage in the development when the side streams 1, 1, 1 
and s, s, s have cut through the hard surface rocks to soft beds beneath and 
lateral valleys, w, w', etc., are being formed parallel with the anticlinal axis. 

Fig. 3. Showing a further stage in the development when the lateral 
stream w has cut through the intervening divides and diverted the drainage 
of r and 1". A second series of lateral streams, W, etc., is being developed 
along the anticlinal parallel with the first series, W, W, etc. The upper portion 
of the synclinal stream, B, B has been diverted by a side stream, R, R. 

Fig. 4. Showing the present drainage system and topography. Both lateral 
streams, w and W, have continued to encroach upon the basins of those adja- 
cent, w resulting in the present Little Wills Creek and W in Big Wills Creek. 
The latter now occupies the axis of the anticlinal throughout its whole length 
and has become the dominant stream of the system under consideration. The 
stream B, B has been further robbed of portions of its drainage basin by streams 
flowing eastward to the Coosa. The synclinal stream T, T has been tapped at 
various points by streams flowing westward into the Tennessee and the drain^ 
age thus diverted from its original course toward the south. — C. W. Hayes. 



2T0 




Lookout Mountain Region, Alabama, illustrating stream adjustment. 
(After E. W. Hayes). 

Approximate scale; i inch = 15 miles. 



STREAM DEVELOPMENT 211 

its waters off by way of /. The lateral stream thus formed {w w. 
Fig. 3) was the beginning of the present Little Wills Creek. The 
same process would have continued till this stream had tapped 
successively all the drainage basins above if it had not en- 
countered a second hard bed through which erosion was very 
slow. After the soft rocks were removed which lay above this 
hard bed, then the same process was repeated which had taken 
place on the original anticlinal ridge; that is, side streams with 
greater fall were able to cut through the hard bed more quickly 
than the main stream and reaching soft rocks below began to 
widen their valleys and then form lateral valleys at these points. 
The same process was repeated with this second set of lateral 
streams. By erosion at the divides D D' D'', etc., Fig. 3, that 
one having the lowest outlet tapped the drainage basin of the one 
adjacent and led its waters off along the axis of the anticlinal. 
The side stream W, Fig. 3, possessed a great advantage over 
any of the others in having a much lower outlet and hence it was 
able to encroach upon them and divert their head-waters to its own 
channel. But with each conquest of new territory, by the addi- 
tional volume of water thus gained, it became more efficient in 
eroding its valley while the streams whose drainage basins had 
been thus diminished were even less able to hold their own. 
Thus the process was cumulative in its effects, and finally the 
stream last formed became the dominant one of the drainage 
system. 

" This process by which the ridge was removed and the streams 
shifted from their original position in the synclinal trough to the 
axis of the anticlinal may be further illustrated by the diagram 
[Fig. 21], representing a section through the ridge and adjacent 
troughs. The heavy line represents the present surface and the 
unbroken lines the beds of rock as they exist at present, below the 
surface. The curved, dotted lines represent the position of the 
beds as they originally existed before their removal. Two of these 
beds, CI and Sr, which are sandstone, offer much greater resistance 
to erosion than the limestones, Cb, Sc, and Sk. The upper curved 
line represents the profile of the land surface on which drainage 
originated, corresponding in position to the line M N in Fig. i 



212 RIVERS OF NORTH AMERICA 

[Plate XI.]. B and T indicate the position of the main streams in 
the synclinal troughs into which the side streams flowed from the 
intervening anticlinal ridge. As already explained, the first 
point at which the upper hard bed C 1 was cut through was on 
the steep slopes of this ridge, as at w and v, where the lateral 
valleys were subsequently formed in a direction parallel with the 
ridge. The upper broken line, then, will represent the surface 
at the second stage of its development, represented in Fig. 2. 
Continuing to erode their channels downward through the soft 
rocks Cb, the streams encountered the second hard bed Sr, and 




Fig. 21. Section through Lookout Mountain, Wills Valley, and Sand Mount- 
ain ; Showing Profile of the Land Surface at Four Stages in the Develop- 
ment of the Present Drainage System. (After C. W. Hayes.) 

the process above described was repeated, producing the surface 
represented by the lower broken line. This is the third stage in 
the development of the drainage system in which the streams had 
the positions indicated in Fig. 3. Finally, the lateral stream 
which started at W, being already through the two hard beds, 
easily distanced its competitors and robbed them of successive 
portions of their drainage area until it became the dominant 
stream of the system. Big Wills Creek. 

** The stream B B Fig. i, which originally flowed the whole 
length of the Lookout synclinal trough, was not permitted to re- 
tain that position. Robbed of all its western tributaries by the 
process above described, it was unable with its diminished volume 
to lower its channel sufficiently fast for its own protection. At a 
point on the eastern side of the synclinal a stream, R R, Fig. 3, 
has cut back from the valley of the Coosa and diverted the upper 
portion from its original channel. Hence the present course of 
Little River, Fig. 4, follows the synclinal to this point and then 
turns sharply to the south-east by a deep rocky gorge. Other 



STREAM DEVELOPMENT 21 3 

portions have been more recently diverted by Wolf and Yellow 
Creeks. Thus the stream which originally drain,^,d ihe whole of 
the synclinal trough and the slopes of the adjacent anticlinal 
ridges has been robbed of the greater part of its drainage basin, 
and Black Creek alone, a mere remnant of the original stream, 
retains the course which it has followed from the beginning. 
For a short time during the stage represented by Fig. 3, Little 
Wills Creek, w w, was the encroaching stream, but reaching 
a hard stratum, its career of conquest was checked, and the 
stream W, more favourably situated, although last born, be- 
came the dominant stream of the system. In the meantime the 
anticlinal ridge had been entirely removed, and in its place 
a deep valley excavated, while the original stream channels 
were left high up in the synclinals now forming the tops of the 
mountains. 

" This is but one of numerous examples in this region which 
might be followed out in detail to show how the drainage system 
has adjusted itself to the structural surface and how the present 
position of the streams is dependent on the dip of the strata and 
the alternation of hard and soft rocks. 

'' It must be borne in mind, however, that only the latest stages 
in the development of the present topography can be followed 
with certainty. The condition which immediately preceded the 
present is easily inferred, but as the processes are followed back- 
ward they become more obscure, and finally are only to be con- 
jectured. Hence in the development of Wills Valley, as it has 
been sketched above, the explanation becomes more largely pure 
hypothesis as the more remote stages are reached. The explana- 
tion offered for the earliest stages is not the only one possible, 
but is perhaps the most probable of a number which might be 
given. 

" Also in the above sketch several complicating factors have 
been purposely omitted for the sake of greater simplicity in pre- 
senting the essential points of the theory. Thus the anticlinal 
arch probably continued to rise during the process of erosion, 
and the effect which this may have had on the resulting topography 
has not been taken into consideration." 



214 RIVERS OF NORTH AMERICA 

The explanation accompanying Plate XI, also borrowed 
from Hayes's report, will assist the reader in understanding 
the bit of ancient history just outlined. 

An analysis of stream adjustment, in a region of folded 
rocks similar to that just presented, was published in 1889, 
by Davis ' in discussing the origin of the peculiar topo- 
graphy of Pennsylvania. The paper just referred to, the 
first ever published that offered a rational explanation of 
the development of the intricate Appalachian drainage, 
should be read by all students of the events in the earth 
history here discussed. 

EFFECTS OF ELEVATION AND SUBSIDENCE ON STREAM 
DEVELOPMENT 

The effect of a general rise of the land throughout a broad 
region is to increase the gradients of the streams, and hence 
to give them greater velocity and greater corrading power. 
If subterranean forces affect the surface in such a manner as 
to tilt a broad area, the streams flowing in the direction of 
downward tilting, other conditions remaining unchanged, 
will have their energy increased, and will therefore deepen 
their channels. This will give their lateral tributaries, 
coming to them more or less nearly at right angles, increased 
energy by lowering their mouths, and still other results will 
follow. 

The reader, no doubt, has already reached the con- 
clusion that a river system is similar to a delicately ad- 
justed machine. A change in the adjustment in any one 

' W. M. Davis, " The Rivers and Valleys of Pennsylvania," in the National 
Geographic Magazine, wo\. i., pp. 183-253. 



STREAM DEVELOPMENT 21 5 

part, or in the complex and far-reaching conditions on which 
stream Hfe depends, necessitates changes throughout large 
portions and perhaps the whole of the system. Of the 
■changes which interfere with the regular and systematic 
development of streams, none are more common or have 
produced more conspicuous results than movements in the 
rocks resulting in upheavals or depressions of the surface. 

Some of the effects of uplift and subsidence of portions of 
the earth's crust have already been considered in connection 
with the discussion of the origin of stream terraces, and of 
the adjustment of stream to geological conditions. There 
are many other results of such changes, however, which are 
of interest to the geographer and geologist. 

Some of the Effects of Elevatioft, — A rise of a region ad- 
jacent to a shallow sea would bring a portion of the sea- 
floor above water, and thus increase the area of dry land. 
The length of the streams entering the sea in the area thus 
affected would be increased, and necessitate a readjustment 
of grade for a long distance up their courses. The lengthen- 
ing of a river by the addition of a broad strip of new land to 
the border of a continent might necessitate such a readjust- 
ment of grade by deposition, in order to enable the stream 
to carry its burden to the sea, that much of the increase in 
energy due to the upheaval would be counteracted so as to 
cause the stream to aggrade its channel at localities where 
corrasion was previously in progress. Elevation, it may 
thus be shown, is not always immediately followed by 
deeper cutting, as might at first be inferred. 

An elevation of the land of the nature just considered, 
tends in general to straighten coast-lines, for the sea bot- 



2l6 RIVERS OF NORTH AMERICA 

torn, as a rule, is more even than land areas, and to trans- 
form previous bays and estuaries into river valleys. 

Increased complexity in the influence that elevation has 
on stream development occurs when, instead of a tilting of 
broad areas along a single axis, the rocks are folded, as in the 
early history of the Appalachian Mountains, so as to produce 
many lines of elevation. In certain regions, especially in 
the central portions of continental areas, the rocks have 
been elevated into vast domes by forces acting from below 
upward. Examples of such domes, and of their influence on 
drainage, are furnished by the Black Hills of Dakota and 
other similar uplifts in the eastern part of the Rocky 
Mountain region. 

Again, throughout broad belts of the earth's crust the 
rocks have been broken by lines of fracture, and the blocks 
thus formed variously tilted and displaced. The displace- 
ment of the broken edges of the same bed, on the opposite 
sides of such a break, is in many instances thousands and 
even tens of thousands of feet. The Great Basin region 
illustrates, more clearly than any other portion of North 
America, the influence of such faults, as they are termed, 
on the relief of the surface. 

The changes in topography produced by faulting probably 
progress slowly with many intermittent movements. If 
the growth of the faults is so slow that streams flowing 
across the affected region can deepen, or aggrade, their 
channels as rapidly as the changes occur, the streams may 
hold their right of way and deepen or fill their channels as 
rapidly as the land is elevated or depressed. When, for 
example, a fault or fold rises athwart the course of a river^ 



STREAM DEVELOPMENT 2\J 

it may corrade its channel as rapidly as the land rises and 
excavate a trench through the growing mountain. Illustra- 
tions of such a history are furnished by the Columbia and 
many of its branches, which cut through the upturned edges 
of fault blocks and produce water-gaps. 

Should the growth of a mountain formed either by a 
folding of the earth's crust, by the elevation of a dome, or 
the growth of an escarpment due to faulting athwart the 
course of a river, be more rapid than it can deepen its 
channel, evidently it will be broken in two. Its lower 
course will be beheaded, its upper course reversed, or its 
waters ponded so as to form a lake. 

When the normal development of a drainage system is 
interrupted by such changes as have just been instanced, a 
new adjustment is made, and the process of development 
continued under the changed condition. 

The broad conclusion reached in reference to the effects 
of upheaval on drainage is, that land is raised above the 
sea by movements in the earth's crust, due principally to 
the cooling and consequent shrinking of the earth's hot 
interior, and the adjustment of the cooler and more rigid 
crust to keep in contact with the continually shrinking 
central mass on which it rests; and streams and other de- 
nuding agencies cut away the areas thus raised. There 
is a continued warfare between these two contending series 
of agencies. 

Some of tJie Effects of Subsidence. — One of the most con- 
spicuous effects of a downward movement of the land on 
the streams draining it and on the valleys that the streams 
have made, is to be seen when coastal plains crossed by 



2l8 RIVERS OF NORTH AMERICA 

large rivers are submerged. In such instances the sea enters 
the river valleys and converts them into bays and estuaries. 
Ridges between adjacent valleys then become capes or pro- 
montories, and the more isolated peaks are perhaps con- 
verted into islands. Examples of conspicuous changes due 
to subsidence are furnished by the Atlantic and St. Law- 
rence drainage slopes. The mouth of the Hudson, as is 
well known, was formerly some seventy miles eastward of 
Long Island, but now, owing to a depression of the land, 
the tide rises and falls at Troy. The St. Lawrence formerly 
discharged to the eastward of what is now Nova Scotia, 
but at present the trunk of the river is shorter by a thou- 
sand miles, and tide-water reaches nearly to Montreal. 

The St. Lawrence below Montreal and the Hudson (Plate 
XV.) below Troy illustrate what is termed a drowned river. 
The former rivers have been betrunked by subsidence. 
Other illustrations of the same occurrence are furnished by 
Delaware and Chesapeake Bays, which are estuaries formed 
by the drowning of river valleys. Chesapeake Bay is also a 
typical example of the way in which a river system may be 
dissected by subsidence. The trunk of the stream has been 
lost by drowning, and several of the former branches enter 
independent estuaries. 

Many other illustrations of similar geographical changes 
are furnished by the coast-lines of North America. The 
ragged coast of Maine, with its multitudes of capes and 
bays and its fringe of islands, owes much of its picturesque- 
ness to the fact that a rough land-surface has been depressed 
so as to allow the sea to encroach upon it. In this in- 
stance, however, the land was formerly covered by glacial 



STREAM DEVELOPMENT 



219 



ice, and in part the roughness of its surface is due to hills 
and ridges produced by glacial deposition. 

On the Pacific coast 
a partially drowned 
river-valley furnishes 
the magnificent bay 
opening to the sea 
through the Golden 
Gate. Puget Sound 
and the fringe of isl- 
ands adjacent to an 
extremely irregular 
coast, northward to 
Lynn Canal and Gla- 
cier Bay, Alaska, al- 
so show the effect of 
the sea entering the 
depressions on the 
border of a continent 
and leaving the hills 
and mountains rising 
above a certain level 
exposed to the air. 
Here again, as on 




the Maine coast the ^^^' ^^' — Map of Chesapeake Bay, Showing by 
r u 1 Heavy Lines the Way in which Various Streams 

roughness of the land ^^uld Unite to Form a Single Trunk-Stream if 
is due in part to ^^e Land were Elevated. (After R. S. Tarr.) 

glacial action. Puget Sound and the similar depression 
northward for a thousand miles were formerly occupied by 
glacial ice, which lingered in the depressions, and allowed 



220 RIVERS OF XORTH AMERICA 

deep accumulations of stream and glacial-born debris to be 
deposited around it. When the ice finally melted, the pres- 
ent tide-ways were left unfilled. 

In general, land exposed to the atmosphere is rendered 
rough and uneven by stream corrasion up to a certain stage 
in its topographical development, and then the prominences 
are removed, the relief becomes subdued, and a plain is the 
ultimate result. If partial submergence occurs during the 
earlier stages of topographical development, a ragged coast- 
line results. Beneath the sea-level the detritus washed 
from the land, together with shells and other material of 
organic origin, is deposited, and inequalities in the bottom 
filled. A rise of the sea bottom, therefore, as previously 
stated, tends to produce even coast-lines. 

The action of the waves and currents of the ocean on its 
shores is analogous in many ways to the action of rivers on 
the bottom and sides of the valleys through which they 
flow, and analogous topographic changes result, which are 
of great scenic as well as geographic importance. Another 
branch of geographic study full of interest and novelty, is 
here open to the student, but at present we must deny our- 
selves the pleasure of a stroll on the salt-sea strands and 
return to the consideration of river development. 

The topographic changes resulting from the drowning of 
a river valley, owing to a subsidence of the land adjacent to 
the ocean, find a counterpart along the shores of lakes 
when their waters rise. In the case of lakes without out- 
lets, as Great Salt Lake, Utah; Pyramid, Winnemucca, and 
Walker Lakes, Nevada, etc., a climatic change, resulting in 
an increased rain-fall or a decrease in evaporation, would 



STREAM DEVELOPMENT 221 

cause the lakes to rise and flood the lower portions of low- 
grade tributary stream-valleys. The extension of the 
waters of Great Salt Lake into the channel of Bear River at 
the present time is an illustration of the drowning of a river 
by this process. The many changes of level experienced by 
the Laurentian lakes, owing to a tilting of their basins, has in 
some instances led to the submergence of river valleys. For 
example, at a time when the waters in the western part of 
Lake Ontario were lower than now, Niagara River exca- 
vated a channel leading past Lewiston, which, owing to a 
tilting of the lake basin, was subsequently flooded. Other 
geographical changes, produced by fluctuation of water-level 
in the lakes referred to, illustrate on a comparatively small 
scale the modifications in coast-lines which result from 
movement in the land. 

SOME OF THE INFLUENCES OF VOLCANIC AGENCIES ON 
STREAM DEVELOPMENT 

Important modifications in stream development have re- 
sulted in numerous instances from interruptions due to 
volcanic agencies. The influences of volcanoes on the 
histories of streams are diverse, but only a few of the more 
pronounced changes of this nature need be noted at this 
time to enable the reader to recognise others without the 
aid of suggestions. 

Volcanoes frequently emit streams of lava which flow 
over the surface of the land in obedience to gravity, and 
take the most favourable courses available. A lava stream 
sometimes flows down a previously eroded valley, and on 
cooling and hardening leaves it partially or wholly filled 



222 RIVERS OF NORTH AMERICA 

with resistant rock. The adjustment of the stream is thus 
greatly disturbed, or perhaps an entirely new start in chan- 
nel-making initiated. Lava streams sometimes cross river 
valleys and dam them so as to form lakes. Instances of 
such a nature are described in the author's book entitled 
Volcanoes of North America, 

Vast lava inundations have occurred in the history of 
North America, as when the Columbia lava of Washington, 
Oregon, Idaho, and California was spread out in successive 
sheets over an area of between two hundred thousand and 
two hundred and fifty thousand square miles and attained a 
thickness in certain regions of over four thousand feet. In 
such an instance, the topography of the inundated region is 
entirely obliterated, the lives of its streams are terminated, 
and a new start in stream development has to be made on 
the surface of the lava covering. The streams from neigh- 
bouring mountains are dammed, lakes are formed which dis- 
charge perhaps across the lava plain, as in the instance of 
Snake River, and a canyon is carved. In the case of the river 
just mentioned, the outspreading of a vast lava plain over the 
region across which it previously flowed, was in a measure 
equivalent to an elevation of the land in its lower course. 

Volcanoes also discharge projectiles in the form of scoria, 
bombs, small rock fragments (Japilli), and dust, which ac- 
cumulate about the vents from which they came and build 
up elevations, usually conical in form, or are carried away 
by the wind, as in the case of volcanic dust, and widely dis- 
tributed. Deposits of this nature also modify the normal 
development of streams, and may even terminate their 
existence. 



STREAM DEVELOPMENT 223 

Volcanic eruptions are frequently accompanied by earth- 
quakes, which cause fissures to open in the soil and rocks, 
and in this manner other accidents, as they may be termed, 
to river development come about. Earthquakes are also 
accompanied by changes of the level of the land, perhaps 
as a cause or as an effect of the changes to which the shocks 
are due, but the influence of such movements in stream de- 
velopment is not essentially different from that produced 
by the changes in elevation already considered. 

Another method by which volcanoes influence the lives 
of streams is through the action of the acids they emit. 
This chemical phase of the subject has received but slight 
attention, but it is evident, from the vast quantities of 
various acid gases poured out during eruptions and even 
for centuries after a volcano has passed to the condition of 
a fumarole or a solfatara, that the chemical action of surface 
waters must thereby be enhanced. 

Volcanoes also exert an influence on climate both on ac- 
count of the heat emitted and the vapours poured into the 
air, and again, by reason of their influence on air currents, 
etc., when prominent mountains are built. All of these 
changes in what may be termed the geography of the air, 
as well as of the relief of the land, exert an influence on the 
history of streams. 

SOME OF THE MODIFICATIONS IN STREAM DEVELOPMENT 
DUE TO CLIMATIC CHANGES 

To appreciate the effects of climatic changes on normal 
stream development, perhaps the best method is to have in 



224 RIVERS OF NORTH AMERICA 

mind an example of a river with numerous branches which 
has become well adjusted to the structure of the rocks 
underlying its hydrographic basin, and then postulate 
atmospheric changes and see what variations in the be- 
haviour of the stream would necessarily result. The con- 
clusions reached by this process of mental analysis may 
be checked by making comparisons between the changes 
thought to logically follow certain climatic variations and 
the condition of the streams in regions where similar 
changes have actually occurred. 

Of the elements in the highly complex atmospheric con- 
ditions embraced in the term climate, those having the 
most direct and tangible influence on the behaviour of 
streams are precipitation and temperature. 

Variations in Precipitation, — An increase in precipitation 
in a region where the mean annual temperature is such that 
all of the water coming to the earth is in the form of rain, 
or if snow falls during a part of the year it is all melted be- 
fore the next succeeding winter, is to increase the volumes 
of the streams and enable them to carry on their work more 
rapidly. 

An increase in rain-fall, other conditions remaining the 
same, means, then, that more debris is transported, and 
when the relation of load to velocity is such as to favour 
corrasion, the stream channels are deepened more rapidly 
than previous to the postulated change. Increased corra- 
sion in the branches of a drainage system, however, means 
usually more rapid deposition, or aggrading, in those por- 
tions of the main stream, or in its branches, where the swift 
up-stream reaches deliver their loads to less rapidly flowing 



STREAM DEVELOPMENT 22$ 

waters. On the whole, streams carry on this work more 
rapidly, and lower their drainage basin to baselevel more 
quickly, under humid than under rainless skies. That is, 
increased volume favours corrasion and hastens the coming 
of the final stage in the history of streams, when they have 
reduced their basins to baselevel. There may be a check in 
this increased activity, however, due to greater rain-fall, 
through the influence of more luxuriant vegetation. 

The effects of a long-continued or secular decrease in pre- 
cipitation on stream development and erosion are illustrated 
on a small scale each summer in the eastern portion of the 
United States and Canada. After the spring rain, as is well 
known, the monthly precipitation greatly decreases, the 
streams become less and less in volume ; many of the weaker 
ones disappear, their supplies diminish, their waters are 
evaporated, or absorbed by the material over which they 
flow. When one follows up a dry stream-channel in late 
summer he comes at length in many instances to where it is 
still occupied by water. The trunk of the drainage system 
has disappeared and the branches fail to unite. The streams 
have been betrunked by a decrease in water supply and an 
increase in loss by evaporation. The weakened streams 
fail to abrade their channels where active corrasion was in 
progress a few months previously, and all the debris carried 
by them is deposited within their channels. 

On account of decrease in load, however, many streams 

are enabled to corrade their channels in certain tracts 

during the dry season, where previously overloading led to 

deposition. 

The changes experienced by streams in a region where 
15 



226 RIVERS OF NORTH AMERICA 

seasonal changes occur, as just referred to, illustrate tlie 
more marked results of secular climatic changes. If precipi- 
tation slowly decreases century after century, even the larger 
rivers may fail to reach the sea, but annual pulsations in 
their branches and even in the main channels may still con- 
tinue. The channels of the main streams and of many of 
their tributaries will then become deeply filled. Some of 
the branches will fail to reach the main stream-channel, and 
the drainage tree, as it appears on a map, is betrunked. 

Many streams illustrating an advanced stage in the shrink- 
ing and betrunking of once vigorous lines of drainage, owing 
to increased dryness of climate, occur in the arid portions 
of the United States, between the Rocky Mountains and 
the Sierra Nevada, and also in Texas and New Mexico. 
An examination of a fairly good map of these regions will 
show many branches of previously extensive rivers, which 
end without uniting so as to form trunk streams. These 
lost rivers furnish illustrations of the dissection of streams 
by a secular decrease in rain-fall and general desiccation. 

In other instances, as in the case of Humboldt River, 
Nevada, a trunk stream is formed from year to year during 
the rainy season, which discharges its waters on to a broad 
desert valley where they spread out in a saline lake and 
are evaporated. During the next succeeding summer, the 
trunk stream fails to reach the lake into which it previously 
discharged and shrinks in volume throughout, leaving 
isolated ponds in the deeper portions of its channel; its 
branches then correspond to the branches of a fallen tree 
from which the trunk has been removed. 

An instructive phase of the drainage of an arid country is 



STREAM DEVELOPMENT 22/ 

furnished when a stream rising in more humid and usually 
mountainous regions, flows across it, and has sufficient vol- 
ume to sustain the losses due to evaporation, and maintains 
its course to the sea. Under such conditions a river may 
continue to deepen its channel through an arid country with- 
out developing branches in its middle and lower courses. 
Such a stream, returning to the analogy between the outline 
of a drainage system and a tree, resembles one of the mon- 
archs of a forest having a tall trunk without branches until 
the spreading crown of foliage is reached. An example of 
such a river, with an immensely long trunk from which 
lateral branches have fallen away, is furnished by the Colo- 
rado. The trunk of the Nile measures about eleven hundred 
miles to the first branch. 

The character of the trunk of a river flowing through an 
arid country will depend largely on the altitude of the land 
through which it flows. If the region is an elevated table- 
land, as in the case of the Colorado, it will corrade its chan- 
nel down to baselevel and form a deep canyon, for the reason 
that general degradation in an arid climate is greatly re- 
tarded. The walls of the canyon will be precipitous because 
the stream deepens its channel more rapidly than its broad- 
ening cliffs are eroded. If, however, the arid region through 
which the river flows is low, aggrading will take place. 

Between the two extremes just cited, many intermediate 
phases may be discovered, dependent mainly on the height 
of the land above the sea. When the topography of the 
region through which flow such tall drainage trees, as we may 
venture to call them, is more carefully studied, the influence 
of many secondary conditions such as rock texture, charac- 



228 RIVERS OF NORTH AMERICA 

ter of the infrequent storms, nature of the vegetation, etc, 
will furnish interesting subjects for investigation. 

Variation in Temperature. — If we dissect out, as it were, 
the effects of secular variations of temperature on the de- 
velopment of streams, from other climatic conditions, we 
find that an increase of temperature promotes evaporation 
and thus decreases the volume of streams; a gradual lower- 
ing of the mean annual temperature for a series of decades, 
or centuries, will favour an increase in the volume of streams. 
But secular variations in temperature mean a profound 
change in other climatic conditions, and start a wave which 
is felt also throughout the organic kingdom in the region 
affected. The resultant changes in fauna and flora react on 
the inorganic kingdom, and notably on the volume, varia- 
tions, and development of streams. An increase in mean 
annual temperature in many regions is followed by a greater 
luxuriance of the flora; while over other extensive areas, as, 
for example, in arid and desert countries, it means a decrease 
in the previously scanty vegetation. A reverse wave of solar 
energy means in general a decrease in the luxuriance in the 
flora of previously warm and humid regions and an increase 
in the vegetation of previously hot and arid regions; while 
cold, humid regions will become still more arctic and 
arboreal vegetation decline. Some of the influences of 
plant-life on the behaviour and growth of streams have 
already been noted and others will be considered. 

The variation in temperature which most directly affects 
the streams is a change from temperate to arctic conditions. 
A lowering of the mean annual temperature in regions 
which previous to the change had a temperate or sub-arctic 



STREAM DEVELOPMENT • 229 

climate, means an increase in the length of the winters and 
a shortening of the summers. A progressive and cumula- 
tive change in this direction would be accompanied ordinarily 
by an increase in the winter's snow-fall and the preservation 
of the snow until late in the spring or even far into the 
summer, and when the accumulated snows finally yielded to 
the increasing temperature of the warm season, floods would 
result. While the ground was frozen and snow-covered, 
erosion would be nil, the streams clear and ice-bound, and 
all the varied processes of corrasion and deposition greatly 
retarded. With the breaking of the river ice when warm 
rains melted the snow, the increased volume of the streams, 
a generous supply of debris loosened by the winter's frost, 
and yet other conditions would favour stream work. In 
many ways, therefore, a moderate or at least not excessive 
lowering of the mean annual temperature, especially in pre- 
viously temperate and sub-arctic regions, would be accom- 
panied by increased activity in stream development. A 
more excessive refrigeration would lead to the accumulation 
of perennial snow and the formation of glaciers. 

Fhictiiations of Streams, — The amount of water flowing 
through most stream channels is subject to many fluctua- 
tions. The most constant streams are such as"- are fed by 
large springs or by the overflow of broad lakes, for the 
reason that, in instances of this nature, the water comes 
from reservoirs of such size as not to be materially influenced 
by sudden rains or by ordinary droughts. The subterranean 
reservoirs referred to as supplying springs are not usually 
definite water-bodies like the lakes in caverns, but masses of 
porous soil and rock which became saturated by percolation. 



230 .RIVERS OF NORTH AMERICA 

It is to be noted, however, that streams flowing from 
large lakes do experience some variation in volume, as when 
a strong wind blowing over a lake causes its waters to rise 
on the side towards which the wind blows ; when this side 
is the one from which the water escapes, a sudden rise is 
experienced in the draining stream. A reverse direction of 
the wind would cause a diminution in the volume of the 
outflowing water. There are also other atmospheric 
changes, such as those producing sudden variations in the 
level of lake water, known as seicheSy which at times have a 
temporary influence on the volume of streams flowing from 
lakes. 

The principal influences which cause streams to vary in 
volume, although diverse and frequently complex, may be 
provisionally classified in three groups, namely, seasonal 
changes of climate, weather changes, and the breaking of 
dams. There are, besides, and perhaps most important 
of all, secular climatic changes, like those that accompanied 
the Glacial epoch, to which the greatest variations in the 
histories of streams are due, but these are beyond our 
immediate dreams. 

Of the changes in climatic conditions accompanying the 
orderly maTch of the season, those producing alternate wet 
and dry periods have the most direct and conspicuous influ- 
ence on the volume of the streams. This, however, is a 
matter of every-day knowledge, and need claim but little 
space at this time. 

Throughout nearly the whole of North America, the rainy 
season is also the cold season ; the ground is frequently 
frozen, thus checking percolation, and the country over 



STREAM DEVELOPMENT 23 1 

large areas is snow-covered. When a thaw occurs, the 
water formed by the melting of the snow, increased perhaps 
by a copious rain-fall, flows quickly over the frozen soil and 
causes floods in the valleys. 

At times when the ground is not frozen, a portion of the 
surface water flows directly to the stream, while another, 
and frequently the larger portion, sinks below the surface 
and percolates slowly away. Under these conditions, there 
are commonly two well-marked periods of rise in the adja- 
cent streams ; the first and quickest rise following the 
inrush of surface run-off, and the second usually involving 
a greater volume of water, caused by the sub-surface water 
percolating out from the sides of valleys and flowing from 
springs. There is marked difference between these two 
floods referred to in various regions, dependent on the 
permeability of the soil and of the rock immediately be- 
neath. When the material composing the hills and valley 
sides is deep and porous, the first rise is small, unless the 
rain-fall is remarkably heavy, and quickly subsides, while 
the slower rise which follows is of longer duration and of 
much greater volume. In regions with an impervious 
covering, such as clay, the first rise of the stream, after a 
heavy rain-fall, is marked both by its suddenness and by the 
large volume of the run-off. The second rise may then be 
slight, or even not recognisable. 

Each of the two conditions of soil referred to is ac- 
companied by characteristic topographic forms. In regions 
covered by impervious soil, as in the case of very large 
areas in the South Atlantic States where residual clays pre- 
dominate, the land is everywhere trenched by stream 



232 RIVERS OF NORTH AMERICA 

channels, which ensure a quick escape for surface water. 
On the other hand, where deep, porous soils occur, as in 
much of the formerly glaciated region of the United States, 
the surface waters are quickly absorbed, and percolate 
slowly away, leaving broad areas between the large stream- 
cut valleys without rill marks. 

Floods in streams are caused by seasonal changes in rain- 
fall and also by the melting of the snow on high mountains. 
In early summer, for example, when the snow is melting on 
the Rocky Mountains, the Missouri and the lower Mississippi 
undergo what is known as the June rise. Rain-fall floods 
follow excessive rains and may occur in a part or the whole 
of a river system. When the swollen tributaries of a trunk 
stream deliver their waters to the main channel at the same 
time, great floods occur below where the branches unite. 
This class of floods can be readily predicted when a suffi- 
cient number of weather records and of gauge readings in 
the principal tributaries are available. Valuable work in 
this connection is being done by the United States Weather 
Bureau. The floods of the Ohio are of this character, but 
the highest rises are augmented by the melting "of winter 
snow. The highest waters occur usually in February, when 
a height of over fifty feet above the summer stage frequently 
occurs. 

The waters of streams are in many cases held in check by 
glacial dams, ice-gorges, accumulations of drift-wood, land- 
slides, avalanches, etc., until the breaking of the obstruction 
permits of the emptying of the reservoirs above them, and 
floods sometimes accompanied by great loss of life and de- 
struction of property occur in the valleys below. Floods of 



STREAM DEVELOPMENT 233 

this character can seldom be predicted for any consider- 
able time in advance, and to this fact is due much of the 
destruction wrought by them. 

Inundations of portions of valleys and flood-plains, not 
usually in danger during annual high-water stages of the 
streams flowing through them, occur when the necks of 
land left by the migration of streams are cut through and a 
new channel is rendered available. The aggrading of stream 
channels on alluvial cones and deltas leads to similar results. 
The clogging of stream channels, owing to abundant deposi- 
tion, also causes them to overflow their banks, especially 
during the annual high-water stages. 

River floods are now being systematically studied in the 
regions occupied by the more civilised nations, and most 
valuable results may be expected fro'm these scientific in- 
vestigations, particularly in the direction of predicting when 
floods are to be expected, thus allowing opportunities 
to counteract their destructive effects, and to utilise the 
inundation of farm lands so as to enrich them by the 
silt thrown down. Much of the richest land in the world 
is situated on the flood-plains of great rivers. These 
plains owe their origin, as has been already explained, 
to the deposits made during high-water stages of the 
streams. In general, it seems better for man to adapt 
his industries to these natural conditions and endeavour to 
utilise the floods to enrich his land, rather than to build 
levees for the purpose of preventing inundations. This 
branch of river study falls properly in the field of the en- 
gineer, and is too technical to be treated at length at this 
time. 



234 RIVERS OF NORTH AMERICA 

SOME OF THE INFLUENCES OF GLACIERS ON STREAM 
DEVELOPMENT 

The origin, growth, and retreat of glaciers in a region pre- 
viously occupied by streams have a varied influence on their 
lives. In the case of alpine glaciers originating about 
isolated peaks or on the summits of mountain ranges, the 
previously excavated stream-valleys become avenues of ice- 
drainage, and in the more elevated portions of such valleys 
all stream work is stopped. Subglacial streams originate 
which are heavily charged with silt, and, judging from 
existing examples, would be in a condition to abrade the 
rocks over which they flowed. The streams below the ends 
of the glaciers, and supplied by their melting, would be 
swollen in summer and greatly diminished in volume in 
winter. Their efficiency as transporting and corrading 
agencies would thus be increased beyond the power they 
would have if the water supplied to them was delivered with 
more uniformity. ' But the chief effect of such a change as 
has just been postulated, as is shown by the study of exist- 
ing glaciers, would, in general, be the overloading of the 
streams below where the drainage changed from a solid to a 
liquid form. Alpine glaciers normally deliver to their drain- 
age streams more debris than the latter are able to transport, 
and aggrading begins at their very sources unless the stream 
channels are remarkably precipitous. Many valleys at the 
present day, which are occupied by glaciers of the alpine 
type in their upper courses, are deeply filled with debris all 
the way from the lower extremities of the glaciers to the 
lake or sea into which they discharge. Broad flood-plains. 



STREAM DEVELOPMENT 235 

bifurcating streams, and valley sides without terraces are 
some of the more striking features of valleys which have 
experienced such changes. 

The modifications in the behaviour of streams flowing 
from an isolated peak or mountain range on which glaciers 
have been developed at the heads of previously excavated 
stream-valleys, are similar to the changes in the lives of 
streams draining a region where a continental glacier 
originates and expands. 

As a continental glacier advances and occupies a pre- 
viously well-drained region, the streams are obliterated over 
most of the area that becomes occupied by the ice. Near 
the outer border of the ice-sheet, however, and perhaps for 
scores of miles back from the margin and beneath the ice, 
preglacial streams may continue to flow or new subglacial 
streams originate. The gradients of these streams will not 
be high, and, judging from the subglacial stream recorders 
in regions occupied by Pleistocene glaciers, will be over- 
loaded and consequently forced to drop debris and raise the 
beds of their channels. When the streams leave the ice and 
expand and bifurcate, as usually happens, their velocities 
will be still further decreased and still more of their loads 
deposited. Flood-plains and alluvial cones merging into 
sand and gravel plains will be the more striking results of 
such conditions. When the streams leave their tunnels in 
the ice, aggrading may be so active that their beds will be 
raised and a check imposed upon the flow of their waters 
through the tunnels from which they bring their loads of 
debris, thus necessitating the raising of the bottoms of the 
tunnels and their enlargement above by melting the ice. 



236 RIVERS OF NORTH AMERICA 

This process, by which ridges of gravel and sand, or osars^ 
are formed beneath ice-sheets both of the continental and 
piedmont types, has been considered, to some extent, in a 
former chapter, and at greater length in a previously pub- 
lished volume by the present writer/ 

When a continental glacier wastes away and its outer 
border retreats, the -zone of marginal deposition recedes 
also, and will be followed in turn by conditions which favour 
corrasion, and the previously formed flood-plains, etc., will 
have channels and valleys cut through them. 

The considerations just presented are necessarily of the 
nature of an outline sketch, but I trust furnish sufficient 
suggestions to enable the reader to fill in the details in the 
case of any special field that may come under his notice. 

SOME OF THE INFLUENCES OF VEGETATION ON STREAM 

DEVELOPMENT 

In a region bare of all vegetation and where the surface is 
inclined, the rain-water gathers quickly into rills and rivu- 
lets which at once begin to excavate channels. When the 
ground is grass-covered, it is protected from the impact of 
the falling drops, and the plant roots bind the soil together 
so as to greatly retard its removal. If shrubs and trees rise 
above a grass-covered surface, still greater protection is 
afforded. The tendency of vegetation is thus to shield the 
land and allow surface water to be absorbed by the soil and 
percolate quietly away, thus robbing it of the power to cor- 

* I. C. Russell, Glaciers of North America, pp. 28, 29, 123-125. Ginn & 
Co., 1897. The name osar was adopted in this volume in place of eskar, kame^ 
or osar, in conformity with American usage. 



STREAM DEVELOPMENT 237 

rade. This is illustrated especially in humid regions where 
ploughed fields are adjacent to meadows, pastures, and 
woodlands. Ploughed fields, if neglected for a few years, 
will become channelled by stream courses, and immature 
•drainage systems developed where the slopes are favourable, 
while adjacent fields covered with grass and other plants 
are not visibly affected. The abandoned fields of Virginia 
and the more southern Atlantic States, cut by thousands of 
gullies, furnish sad testimony of the disasters which follow 
the breaking of the soil in regions where the waters are 
retained at the surface and gather into rills instead of per- 
colating slowly away. If the soil and subsoil are deep and 
porous, however, the fields may not be seriously affected 
by the breaking of the soil. 

While vegetation retards mechanical corrasion, it favours 
the chemical action of percolating waters by supplying them 
with organic acids, as has already been explained, and 
hastens chemical corrasion. 

Vegetation has an effect on streams also, from the fact 
that it promotes evaporation and retards the gathering of 
surface waters in channels. Evaporation is favoured by the 
retention of the water, and yet still more efficiently through 
the vital functions of the plants themselves. Water is 
taken in by the rootlets and escapes from the leaves. The 
amount of water returned to the atmosphere in this manner 
from areas clothed with luxuriant vegetation is surprisingly 
great. It has been stated that to grow a ton of hay the 
grass plants must drink in two to three hundred tons of water.* 

'J. W. Powell, National Geographic Monographs, vol. i., p. 69, published 
by the American Book Co., New York, 1895. 



238 RIVERS OF NORTH AMERICA 

The run-off, as the escape of rain-water by surface streams 
is termed, in plant-covered regions is retarded and the 
volume of the draining streams rendered more uniform 
than it would be should the vegetation be removed. The 
work of streams is thus checked in two conspicuous ways by 
vegetation : first, by the decrease in surface waters due to 
evaporation, and second, by diminishing floods. 

When a mat of vegetation mantles the ground, as espe- 
cially when broad areas are deeply moss-covered, as in much 
of Alaska and about the shores of Puget Sound, the surface 
waters are filtered of practically all material in suspension 
and gather into rills which are clear, although frequently 
amber-coloured on account of organic matter in solution, 
and hence have but slight powers to abrade the rocks over 
which they flow. 

The influence of the roots of trees, and particularly of 
willows, alders, cottonwoods, etc., which flourish along the 
margins of streams, on erosion of the banks is well known. 
Such vegetation assists particularly in retaining the sediment 
of streams when they expand and flood the adjacent land, 
thus assisting materially in the formation of natural levees. 

The evil effects of removing the forests from a mountain- 
ous region are seen especially in the quicker and greater run- 
off of the waters falling on it. This is accompanied by 
more rapid erosion in the upper courses of the stream and 
aggrading in the valleys lower down. Flood-plains are thus 
raised where the gradients of the streams decrease, and pre- 
viously fruitful areas along the borders of the rivers may 
be converted into barren tracts of gravel and sand, which 
remain for a long time unavailable for agriculture. 



Plate XII. 




Fig. a. Beaver Dam, Wyoming. 
(Photograph by W. H. Jackson.) 




Fig. B. Dam of Drift-Wood, Teanaway River, Washington. 



STREAM DEVELOPMENT 239 

The influence of vegetation on stream development, as 
shown even 'by this hasty and incomplete review, is highly 
complex. In fact the influences of vegetation counteract 
each other, as when dense plant growth and a layer of de- 
caying leaves and branches retard mechanical corrasion, but 
by supplying organic acids to percolating waters, promote 
chemical corrasion. 

In forest-covered regions, especially where the ground is 
encumbered with undergrowths, or where mosses and li- 
chens form a thick mantle, the run-off is retarded and stream 
development delayed. When the land is grass-covered, as 
in prairie regions, where in general the rain-fall is moderate, 
erosion is checked. In many instances, no mechanical erosion 
occurs in prairie regions, because all the water that falls is 
absorbed by the deep porous soil and evaporated largely 
through the agency of plants, or percolates slowly away. 

In regions imperfectly clothed with desert shrubs, and 
where the bunch-grass common to such localities does not 
form a sod, as in the thousands of sage-brush valleys of the 
arid regions of North America, the scanty rain-fall is apt to 
come in the form of violent storms and copious downpours. 
The water being unchecked by vegetation gathers quickly 
into streams wherever the surface slope is favourable, and 
rushes along bearing with it heavy loads of debris. Be- 
tween the infrequent storms the stream channels become 
clogged with debris swept in by the wind or slowly creeping 
down their sides under the influence of changes of tempera- 
ture and other agencies. When the next *' cloud-burst " 
comes, perhaps after the lapse of scores of years, the ac- 
cumulated debris is again swept onward. 



240 RIVERS OF NORTH AMERICA 

The conditions most fav^ourable for general mechanical 
degradation and stream corrasion and deposition, so far as 
vegetation is concerned, are when land is bare or but scan- 
tily clothed with plants of any kind. If, in addition, the 
annual rain-fall is spasmodic, instead of the same amount 
of water falling on a given area being evenly distributed 
throughout the year, exceedingly favourable conditions for 
the development of streams result, providing the requisite 
slopes are present. 

These conditions are more nearly fulfilled in the arid, 
south-west portion of the United States than in any other 
part of North America, and it is there that the labours of 
Newberry, Powell, Gilbert, Button, and others have been 
fruitful in such great results in the way of interpreting the 
origin of topographic forms. 

Drift-Wood, — In considering the mechanical work of 
streams from either an engineering or a purely geographical 
point of view, account needs to be taken of the influence of 
the drift-wood carried by them. In many instances, the 
amount of floating timber and of trees lodged against the 
bottom and sides of a stream is so great as seriously to im- 
pede navigation, and not infrequently to render it impos- 
sible. Drift-wood, also, in one way or another, both assists 
and retards erosion, and by diverting streams from their 
channels leads to important geographical changes. 

As is well known, streams flowing through forested regions 
receive great numbers of trees which fall into them princi- 
pally on account of the undercutting and consequent cav- 
ing of their banks. Fallen trees are swept into streams 
during floods, and when ice-gorges occur great destruction 



STREAM DEVELOPMENT 24 1 

of timber sometimes results. In ascending such rivers as 
the Mississippi or the Yukon, one frequently sees whole 
trees, with branches and roots attached, floating with the 
current. In many such instances, the branches or roots 
drag on the bottom and disturb the mud or sand, and thus 
aid in the transportation of rock debris. At times the 
drifting trees become water-logged, and sink to the bottom 
in such a manner as to become anchored at one end, and 
being swayed by the current again assist corrasion by causing 
eddies in the water, as well as by the direct agitation of the 
material in which their roots or branches are embedded. 

Drift-wood carried by swift streams, and especially during 
floods, tends to leave the belt of most rapid flow and ac- 
cumulate along the banks, thus greatly increasing the 
chances of its becoming lodged. Trees that have fallen 
from the bank of a stream, but are yet held by their roots, 
cause obstructions and retard the progress of floating tim- 
ber. In these and still other ways the trees which fall into 
streams have a conservative influence and tend to retard 
lateral corrasion. 

The conservative tendencies of drift-wood are also seen on 
lake and ocean shores where it is thrown on the beach, and, 
becoming partially or wholly buried in sand and mud, serves 
to bind the shore accumulations together and counteract 
the force of the waves. The protection aff'orded by stranded 
drift-wood is especially well illustrated on the borders of the 
Laurentian lakes, and on the beaches of Puget Sound. 
At each of these localities one may frequently walk for 
several miles by stepping from one stranded log to another. 

In each of the instances referred to, however, the natural 

16 



242 RIVERS OF NORTH AMERICA 

accumulation of drift-wood is augmented by the waste from 
saw-mills. 

The process by which the beaches of lakes, sounds, etc., 
are protected by stranded timber is in action also along 
river banks, but the ability of the currents in removing such 
obstructions is then usually more pronounced. 

The conservative influence of drift-wood on river banks is 
well shown at numerous localities along the Yukon, more 
especially where the river divides so as to enclose islands. 
Where small branches of the river, or bayous, leave the 
main stream, as has already been mentioned, there are fre- 
quently large accumulations of stranded drift-wood. These 
** wood-yards ** are in numerous instances several acres in 
area, and from fifteen to twenty or more feet deep. In 
many localities, as has been observed by the writer, the en- 
trances to bayous have been completely closed by these ac- 
cumulations, the interstices between the logs and broken 
branches being clogged with mud and sand, so that practi- 
cally no water enters the side channels except when the river 
is in flood. When the up-stream ends of the bayous are 
dammed in this manner, their down-stream extremities be- 
come silted up and they are transformed into lakelets. The 
accumulation of drift-wood thus tends to confine the river 
to its main channel and retard lateral corrasion. 

In mountainous regions where forests thrive, the streams 
are sometimes completely dammed by accumulations of 
drift-wood and forced to excavate new channels. These 
dams are usually started by the falling of a large tree across 
a stream, which holds drift-wood brought from above. In 
this way a skeleton dam, as it were, is formed, but the 



STREAM DEVELOPMENT 243 

openings are apt to become clogged with smaller branches 
and leaves, and then more or less completely filled with sand 
and mud. Several instances of this nature have come under 
the writer's notice in the Cascade Mountains. The middle 
fork of Teanaway River, Washington, for example, in several 
localities has been completely turned from a former course 
by dams of the nature just described, and caused to ex- 
cavate a new channel. A view of one of these natural 
log-dams is presented on Plate XII. In this instance, a 
former channel has been filled with drift-wood to a depth 
of about twenty feet for a distance of some three hundred 
yards and the stream completely diverted. At the upper 
end of the obstruction, sand and gravel have been deposited 
against the drift-wood to the depth of several feet, and now 
form the bank of the stream where it leaves its former course. 
The most remarkable example of the influence of accumu- 
lations of drift-wood on the behaviour of streams yet 
reported in North America is probably furnished by the 
timber rafts, as they are termed, in Red River, Louisiana. 
These rafts are several square miles in area, and have been 
in existence so long that soil has formed on them in which 
trees have taken root and flourished so as to produce a 
floating forest. It is stated by Humphreys and Abbot ' 
that these rafts contain an immense accumulation of tree- 
trunks, some floating, and others so water-logged as to sink 
and thus still more effectually to block the channel. From 
the rotting of the logs at the lower ends of the rafts and 
fresh accumulations at their upper ends, they are gradually 
migrating up stream. These obstructions tend to pond the 

' Report on the Mississippi River ^ 1861, p. 37. 



244 RIVERS OF NORTH AMERICA 

river and cause it to form lake-like expansions, which some- 
times discharge through new outlets. The influence of the 
rafts on Red River is much the same as in the smaller in- 
stance noted above in the case of Teanaway River, and 
illustrates the disturbances which the streams of forested 
regions frequently experience. 

SUPERIMPOSED STREAMS 

The reader is already aware that a consequent stream 
follows a predetermined course inherited from pre-existing 
surface conditions, and is at first not influenced by the 
structure of the rocks beneath the surface. It also happens 
that a stream may have its course determined by rocks that 
existed above the surface now exposed, but which have been 
removed, and may simulate a consequent stream. 

If we imagine an ice-sheet to have covered a region of 
mild relief, and that streams flowing over the surface of 
the ice were slowly lowered upon the land beneath as the 
ice melted, they might entrench themselves in the rocks so 
as to hold their former courses after the ice entirely disap- 
peared. The streams might have their direction of flow 
determined to some extent by the surface features of the 
land uncovered by the melting of the ice, but they would 
be uninfluenced by the structure of the rocks underlying 
the exposed surface. 

Not only glaciers, however, but geological formations are 
removed from the superficial portions of large areas. 

To present another hypothetical case: imagine a region 
of folded and faulted rocks to have been worn down to a 
peneplain and then submerged beneath the ocean and 



STREAM DEVELOPMENT 245 

covered with a horizontal sheet of sediment. Should such 
an area be again upraised, consequent streams would be 
developed on its surface and begin an orderly sequence 
of changes as they advanced with the task of again reduc- 
ing the land to baselevel. If the buried peneplain was above 
the new baselevel, the master streams would cut through 
the covering, presumably of soft rocks, resting on the old 
peneplain and sink their channels into it. Should the 
covering of soft rock be now removed, the streams would 
flow across the uncovered region in courses determined by 
the surface slope of the cover on which they originated, and 
without reference to the surface features of the exposed 
plain or to the structure of the rocks beneath it. A drain- 
age system inherited in this manner by one geological 
terrane from another is said to be superimposed. 

Such a history as has just been outlined furnishes an ex- 
planation of the geography of certain regions which cannot 
be satisfactorily accounted for in any other way. 

In east-central New Jersey we have a coastal plain some 
twenty-five to thirty miles broad. Rising from this plain 
there are long, narrow ridges of hard igneous rock, such as 
the double ridge known as the Watchung Mountains, the 
Palisades of the Hudson, and several other smaller hills and 
ridges of the same general character. These ridges have 
remarkably even crest-lines, but are notched in places by 
water-gaps and wind-gaps. As has been admirably worked 
out by Davis and Wood,' the tops of these ridges are por- 

' W. M. Davis and J. Walter Wood, " The Geographical Development of 
Northern New Jersey," in Boston Society of Natural History, Proceedings^ 
vol. xxiv., pp. 365-423, 1889. 



246 RIVERS OF NORTH AMERICA 

tions of an ancient peneplain which has been upraised some- 
what irregularly, and eroded so as to leave the edges of the 
sheets of hard rock which traverse it in bold relief. Previous 
to the upraising of the peneplain it was depressed beneath 
the sea and soft sedimentary beds spread over it, which 
reached nearly if not completely across the central part 
of the State. The peneplain with its cover of soft rock 
was then raised and a system of consequent streams came 
into existence. The streams cut through in the soft sur- 
face beds, were lowered to the concealed peneplain be- 
neath, and continued to deepen their channels. The 
streams flowed across the edges of the hard beds and carved 
notches in them. When the soft surface-sheet was finally 
removed and the formerly buried peneplain exposed, some 
of the streams maintained their courses and still flowed 
through water-gaps in the ridges, while others, by the system 
of adjustment to rock structure discussed on a previous 
page, were diverted to easier courses, leaving notches, or 
wind-gaps. 

The process by which streams are inherited by one series 
of rocks from a higher series, is simulated in some of its 
features by a reverse process. When the roof of a cavern 
crumbles and falls in, a subterranean avenue of drainage 
becomes an open channel, and the stream flowing through 
it becomes a surface stream. Such a stream is an inheri- 
tance from a lower series of rocks by a higher series. If a 
name were desired for this minor feature of the drainage of 
certain regions, it might be termed sicbimposed, I believe, 
however, that the nomenclature of geography should grow 
slowly and spontaneously and not be forced. 



STREAM DEVELOPMENT 247 

MIGRATION OF DIVIDES 

When the feeding rivulets of two rivers flow in opposite 
directions from the crest of a mountain range, the Hne 
separating them is termed a water-parting, or, more briefly, 
a divide. In general, then, a divide is the common bound- 
ary between two adjacent drainage systems. In the Rocky 
Mountain region we have the '* continental divide," which 
parts the waters flowing to the Atlantic and Pacific respect- 
ively. 

A divide, however, is not necessarily a mountain range, 
but may be a plateau or a plain. Neither is it a sharply 
defined line, but may be a broad surface and the actual water- 
parting indefinite. For example, a portion of the divide be- 
tween the waters flowing to Red River and thence to Lake 
Winnipeg and Hudson Bay, and those finding their way to 
the Mississippi, is in a region of mild relief and is not only 
broad but varies with seasonal changes. During certain 
seasons when the streams are swollen, there is no true divide, 
the valley being flooded in such a manner that steam- 
boats may pass from the Mississippi to the Red River, or 
in the reverse direction. Thus, at times, direct river navi- 
gation is possible from the Gulf of Mexico to Hudson Bay. 

The divides between adjacent drainage slopes and the 
ridges between neighbouring streams belonging to the same 
river system, although geographically of great importance 
and among the most persistent features in the topography 
of the land — since corrasion along them is reduced to a 
minimum, — are yet, like all other elements in a landscape, 
subject to change. 



248 RIVERS OF NORTH AMERICA 

The laws governing the migration of divides have already 
'been briefly considered in discussing river piracy, but their 
importance is sufficient excuse for some repetition. The 
slopes on the opposite sides of a divide, it is safe to say, are 
never the same. The streams descending the steeper side 
will have the greater velocity and will tend to deepen their 
channels and to extend their branches by backward cutting 
more rapidly than their rivals flowing down more gentle 
slopes, and hence cause what is termed a migration of the 
divide. If other conditions are the same, but the streams 
flowing in one direction from a divide have a shorter course 
to the sea than their opposite neighbours, the task before 
them, in order to cut down their channels to baselevel, will 
be less, and consequently sooner accomplished. Hence, as 
the work of the opposite-flowing stream progresses, the 
divide between them will be shifted toward the one that 
works more slowly. The area drained by the shorter stream 
will be enlarged at the expense of its less active neighbour. 
Again, if the rocks on one side of a divide are softer, or 
more easily dissolved than those on the opposite side, other 
conditions being the same, the streams flowing over the 
softer rock will evidently progress with their task more 
rapidly than those having to cut down their channels in 
more resistant material, and hence will be enabled to 
extend their head branches more rapidly than their op- 
ponents and capture new territory. In other words, the 
divide will be shifted toward the side where the hard rocks 
occur. 

Should the rain-fall on one side of a divide be heavier 
than on the opposite side, the streams on the rainy side will 



STREAM DEVELOPMENT 249 

be larger than on the other side, and, in general, will lower 
their channels more rapidly than their weaker opponents. 
Should inequality in precipitation be the controlling condi- 
tion, manifestly the divide would migrate toward the side 
having the smaller rain-fall. Yet another condition which 
might cause the migration of a divide, if not controlled by 
other circumstances, is the structure of the rocks forming the 
water-parting. If the rocks are in layers and gently inclined 
toward one drainage slope, as is frequently the case in 
ridges due to faulting, the streams descending the longer 
slope will have to remove more rock in order to reach base- 
level than the swifter streams flowing over the broken edges 
of the strata exposed in the steeper side of the ridge. In 
nature, also, the conditions just postulated are usually 
coupled with others which favour the more rapid cutting of 
the edges of the inclined beds. The declivity of a ridge 
formed of inclined beds is usually steeper on the side in 
which the edges of the strata are exposed ; the streams 
on that side are thus given a steeper grade and flow 
more swiftly than their opponents, thus favouring more 
rapid corrasion. If alternating hard and soft beds oc- 
cur, this again favours the work of the streams flowing 
over their broken edges, by allowing them to remove 
the exposed portions of the soft layers, thus undermin- 
ing the more resistant beds and favouring their removal by 
sapping. 

The Sierra Nevada is, in the main, a great monoclinal 
ridge of the character just described. The strata dip west- 
ward and form a long and comparatively gentle slope on that 
side, but present a bold escarpment formed of the broken 



2 50 RIVERS OF NORTH AMERICA 

edges of the strata upraised along a belt of faulting to the 
eastward. The streams flowing westward are larger than 
those descending the steep eastern slope for the reason that 
they drain greater areas, and also because the rain-fall on 
the western is more abundant than on the eastern slope. 
But in spite of these advantages the eastward-flowing 
streams, having steeper gradients, and far less rock to re- 
move in order to cut to the same depth, have been enabled 
to extend their head-waters by backward corrasion so as to 
cut through what was formerly the divide at the crest of the 
range and acquire territory on the -western slope. The 
water-parting is now west of the topographic crest-line of 
the mountain and is still migrating westward. The exist- 
ence of large glaciers on the higher portions of the range, at 
a comparatively recent date, interfered with stream develop- 
ment, but did not change the conditions so far as the 
westward migration of the divide between the Pacific and 
Great Basin drainage is concerned. 

It does not seem necessary to present other arguments in 
order to establish the law that when a ridge dividing two 
drainage systems is composed of inclined beds which slope 
in one direction from its longer axis, other conditions being 
the same, the divide will migrate, as erosion progresses, 
with the slope of the beds. 

The outline presented in the last few pages of the laws 
governing the migration of divides, although brief, is suffi- 
cient to show that the conditions entering into the problem 
are complex. This complexity is still further enhanced 
when the influence of movements in the earth's crust are 
also brought into play. Although, as previously stated, 



Plate XIII. 




Contour Map of a Portion of the Cat.^kill Mountains, N. Y., lUubtrating River Piracy. 
(After N. H. Darton. Topography by U. S. Geological Survey.) 

Approximate scale : i inch = 7000 feet. Contour Interval, 20 feet. 



STREAM DEVELOPMENT 2$ I 

divides are among the most stable of the geographical 
features of the land, they are continually changing. This 
shifting of the position of the lines of parting between op- 
posing drainage slopes is usually exceedingly slow, but 
under certain conditions may be a comparatively rapid pro- 
cess, as will be seen by reverting to the discussion of the 
origin of subsequent streams, and the manner in which they 
extend their channel by headward cutting so as to cap- 
ture rival streams and divert their waters. This process 
leads to great and even rapid changes in the positions of 
divides. 

Another illustration of the migration of a divide may be 
of interest to the reader. In the Catskill Mountains we 
have a table-land sloping gently westward, but presenting a 
bold escarpment about one thousand five hundred feet high, 
facing the Hudson. A portion of this plateau and its east- 
ward-facing escarpment is shown on the map forming Plate 
XIII. The rocks forming the plateau are sedimentary beds 
of hard sandstone and soft shale, which dip gently westward 
and present their broken edges in the escarpment. As has 
been shown by Darton,^ the regularity of the precipitous 
eastern border of the plateau was broken by alcoves and 
recesses, inherited from a preceding geographical cycle, and 
in these embayments eastward-flowing streams originated. 
Of these the Kaaters Kill and Plaaters Kill are the best 
examples. Streams also came into existence on the gentle 
western slope of the plateau and flowed westward ; the head 

' N. H. Darton, *' Examples of Stream-Robbing in the Catskill Mountains," 
in Bulletin of the Geological Society of America, vol. vii., pp. 505-507, Plate 
xxiii., 1896. 



252 RIVERS OF NORTH AMERICA 

branches of one of these, Schoharie Creek, are shown on the 
accompanying map. The conditions are thus especially 
favourable for the processes of stream capture and the 
migration of a divide, already described. 

It is evident from an inspection of the map, that the 
branches of Schoharie Creek were formerly longer than now, 
and carried away the surplus water from the plateau even 
to the edge of its eastern escarpment, but the Kaaters Kill 
and Plaaters Kill have been enabled to extend their head 
branches so as to capture a considerable portion of the pre- 
vious western drainage. The divide has migrated westward, 
and some of the former branches of Schoharie Creek have 
been diverted. This history is brought out so graphically 
on the accompanying map that further explanation seems 
unnecessary. 

One result of the process just considered is shown by the 
direction of flow of the higher branches of the capturing 
streams. Normally the branches of a stream join the main 
trunk at an acute angle, the flow in the branch and in the 
trunk near their place of union being in the same general 
direction. But in the case of the capturing streams in- 
stanced above, their head branches come in at more than a 
right angle; the captured branches maintain the direction 
they had when flowing to Schoharie Creek, and in general 
flow westward, while the trunk streams to which they are 
now tributary flow eastward. Such an abnormal arrange- 
ment of the branches of a drainage tree in any region should 
at once suggest that a recent capture has been made, but 
yet rock texture and other conditions might produce a 
similar result. 



STREAM DEVELOPMENT 253 

The process of stream capture, so admirably illustrated in 
the Catskills, furnishes an example of one method by which 
fishes, mollusks, etc., might be enabled to migrate from one 
side of a mountain range to the other. The opening of 
gaps in the crest of a high ridge or mountain range would 
also facilitate the distribution of plants and animals not de- 
pendent directly on streams for their means of travel. Im- 
portant influences even on the migration of peoples may be 
traced to the same cause.* 

^ Books of reference : 
C Humphreys and Abbot. Physics and Hydraulics of the Mississippi, 
War Department, Washington, D. C, 1861J 

Thomas Russell. Meteorology. Macmillan & Co., 1895. (Chapters IX., 
*' Rivers and Floods," and X., " River-Stage Predictions.") 

Park Morrill. Floods of the Mississippi River. Weather Bureau, Wash- 
ington, D. C, 1897. 



CHAPTER VIII 

SOME OF THE CHARACTERISTICS OF AMERICAN 

RIVERS 

THE many details that have occupied the reader's atten- 
tion in the preceding chapter have perhaps diverted 
attention from certain general conclusions pertaining to the 
lives of streams. A brief review of the leading characteris- 
tics of a few of the rivers of America will possibly correct 
this tendency and at the same time afford an opportunity to 
apply some of the principles stated, perhaps too empirically, 
in what has gone before. 

The initial slopes of large rivers must evidently be deter- 
mined by the slope of the land due to upheaval. In many, 
and probably most, instances, however, the surface slopes 
that gave direction to the youthful streams have been de- 
formed by movements in the rocks of the nature of a tilt- 
ing of the land over broad areas; again, the rocks have 
been folded, or broken and one side of the fracture upraised 
above the opposite side, so as to affect the surface drainage. 
While these changes were in progress in many instances, the 
streams have maintained their positions or right of way by 
deepening their channels as fast as the rocks were raised, or 
by filling in the depressions due to subsidence; but in other 
instances the streams have been reversed or given other di- 

254 



SOME CHARACTERISTICS OF AMERICAN RIVERS 255 

rections, owing to the modifications in conditions referred 
to. The present courses of even the larger rivers do not, 
therefore, in themselves, necessarily record the original 
slope of the land. 

Throughout the lives of streams they have the power of 
extending their branches in a manner analogous to the 
growth of a tree by the lengthening of its terminal twigs. 
This process, as we have seen, leads to rivalry between 
neighbouring streams, and the shifting or migration of the 
boundary line between adjacent drainage areas. Climatic 
changes may also favour the extension of certain drainage 
areas and the diminution of others. For these and still 
other reasons, the boundaries of the original slopes which 
gave the large rivers their general directions have been 
greatly modified and in some instances rendered indeter- 
minate ; yet when the general changes that land areas 
pass through and the laws of stream development are 
known, much of the history of a river system can be 
deciphered. 

Some of the modifications that have taken place in the 
various drainage areas of North America, due to changes in 
the elevation of the land, variation of climate, normal stream 
development, etc., can be recognised even in a general view 
of the present distribution of the streams. Individual 
rivers furnish too small a unit with which to measure the 
greater slopes produced in the surface of North America by 
upheaval, and a better idea of the character the surface of 
the continent would present, had there been no erosion, can 
be had by considering the main drainage areas. It must be 
remembered, however, that the upheavals which established 



256 RIVERS OF NORTH AMERICA 

the main divides occurred at widely separated intervals, and 
that in many instances, as in the Appalachians, the main 
rivers have been persistent through more than one geograph- 
ical cycle. 

DRAINAGE SLOPES 

An examination of any fairly good map of North America 
will show that the continent is divided into nine principal 
drainage slopes. These may be conveniently named, as has 
been done on the map forming Plate XIV., after the water- 
bodies into which their rivers discharge. This classification 
is, in fact, arbitrary, and certain minor or but little-known 
regions, as the north-east coast of Labrador, much of the 
Arctic archipelago, Greenland, etc., are not included. The 
divisions chosen, largely for the purpose of dissecting a vast 
region into its component parts for convenience of study, 
are described below. These descriptions are brief, and 
intended simply to supplement the accompanying map. 

Atlantic Drainage Slope. — This includes the land from 
Florida to Nova Scotia which drains to the Atlantic. The 
principal rivers are the Alabama, Savannah, Roanoke, 
James, Potomac, Susquehanna, Delaware, Hudson, Con- 
necticut, Merrimac, and St. John. 

St, Lawrence Drainage Slope, — The region draining to the 
Great Lakes and Lake Champlain, or directly to the St. 
Lawrence and its tributaries, is here included. 

Hudson Bay Drainage Slope, — This division comprises the 
vast forested area of low relief lying principally in Canada, 
and draining through many valleys to Hudson Bay. 

Arctic Drainage Slope, — Comprising the region north of 



Plate XIV. 




Outline Map of North America showing Drainage wSlopes 

A— Arctic Drainage Slope. Q— Gulf of Mexico Drainage Slope. 

At.— Atlantic Drainage Slope. G B— Great Basin Drainage Slope. 

B— Bering Sea Drainage Slope. H— Hudson Bay Drainage Slope. 

C— Caribbean Sea Drainage Slope. P— Pacific Drainage Slope. 

St. L— St. Lawrence Drainage Slope. 



SOME CHARACTERISTICS OF AMERICAN RIVERS 257 

the Hudson Bay drainage, several of the little-known Arctic 
islands, the basin of the Mackenzie, and northern Alaska. 

Bering Drainage Slope. — Embracing the basins of the 
Yukon, Kuskoquim, and several subordinate rivers, which 
discharge into Bering Sea. 

Pacific Drainage Slope, — The long and comparatively nar- 
row belt of country, which discharges its drainage to the 
Pacific, might be subdivided under the general plan here 
followed, but this does not seem necessary here at present. 
All of the land from the Aleutian Islands to Panama, which 
sends its contributions of surplus waters to the Pacific, is here 
included. The principal rivers are the Copper, Stikine, 
Fraser, Columbia, Sacramento, and Colorado. 

Great Basin Drainage, — The arid region embracing the 
eastern border of California, nearly all of Nevada, Southern 
Oregon, and a large part of Utah, which does not send any 
streams to the ocean, is here included. Similar but sub- 
ordinate interior basins in Mexico are at present neglected. 

Gulf Drainage Slope, — All of the land, including the vast 
hydrographic basin of the Mississippi, which is drained by 
streams flowing to the Gulf of Mexico, is here considered as 
a single drainage slope. 

Caribbean Drainage Slope, — The region from Northern 
Yucatan to the junction of the Isthmus of Panama with the 
South American continent, from which streams flow to 
the Caribbean Sea, forms the most southern of the several 
drainage slopes here considered. 

The above classification, as has been said, is in part arbi- 
trary, but in its main features is believed to indicate condi- 



258 RIVERS OF NORTH AMERICA 

tions which a geographer finds it convenient to have in 
mind. 

The present extent and relationships of the drainage 
slopes noted above are in part due to the movements in 
the earth's crust, and to what may be termed the accidental 
inequalities of the original surface of the land, but not at a 
single period. Each separate province has its own special 
history. In part, also, the boundaries of the drainage 
slopes are dependent on present climatic conditions, as is 
seen in the Great Basin region. Then, too, the character 
of the rocks, whether soft or hard, and the way in which the 
layers composing them are inclined, have had a directing in- 
fluence on the migrations of the dividing lines between 
adjacent drainage areas. To a marked extent, also, the 
boundary lines under consideration have been shifted by 
what is termed stream development, during which one 
stream extends its head-water branches so as to capture 
territory belonging to a neighbouring stream. In several 
great regions in North America, the land has been worn 
down to a peneplain, and then upraised, thus making con- 
spicuous changes in the balance of power among the streams 
of various drainage areas. Again, the divides between the 
drainage slopes in the northern half of the continent have 
been modified by glacial action. Still other complexities in 
the histories of the drainage slopes as we now find them 
will appear later. 

A hasty glance at the great natural divisions of North 
America, as marked out by the directions of drainage, re- 
veals the fact that the study we have undertaken is a por- 
tion of a long and still more highly complex history. In 



SOME CHARACTERISTICS OE AMERICAN RIVERS 259 

treating such a broad subject as the nature, origin, and 
history of the streams of a continent, it is advisable to begin 
with individual streams, and to start, perhaps, with even 
their tiniest branches, and gradually expand our field of ob- 
servation. Each of the drainage slopes enumerated has its 
main rivers fed by many branches. Each trunk stream and 
each individual branch, however small, is an active agency 
which is engaged in producing changes, and each trunk and 
branch of the many drainage systems is modified in its 
action by climatic, geological, and other conditions. 

LEADING FEATURES OF THE SEVERAL DRAINAGE SLOPES 

New England Rivers. — The Connecticut with its charming 
scenery, the historic Merrimac, the forest-bordered Kenne- 
bec, and many other streams in the northern portion of 
the Atlantic drainage slope flow through valleys sunken in a 
tilted peneplain. The general level to which the hills rise 
throughout the lower courses of these streams is itself a 
record of the work of rivers ; for the land, during a geographic 
cycle long since closed, was cut away to near sea-level and 
then bodily upraised and tilted southward. The planation 
by the old streams was not complete, and the remnants of 
the uplands that were left rise as mountains above the gen- 
eral level of the plateau in which the modern rivers have 
entrenched themselves. The portions of the plateau sur- 
face which were once nearly smooth have been roughened 
by the excavation of valleys, leaving the spaces between 
the streams in relief. 

The rivers meander through rich bottom-lands of their 



260 RIVERS OF NORTH AMERICA 

own making. The borders of the valleys are frequently in 
steps or terraces rising one above another; each nearly flat- 
topped shelf furnishes sites for prosperous farms, thriving 
villages, or populous cities. The terraces on the sides of 
the present valleys are formed of gravel and sand, and show 
how deeply the still older valleys were filled and the pro- 
gress that has been made in their re-excavation. 

Not only have the valleys long and varied histories, but 
each roaring cascade and musical rapid, each shadowy pool 
and placid reach of the streams where the water loiters, and 
each graceful bend have a cause for their existence, and an 
instructive and even romantic story to tell. 

A Drowned River, — The noble Hudson, in large part an 
arm of the sea, where the tides rise and fall, divides a 
mountain range. The early history of the river has not 
been fully traced, but apparently it had its course defined 
before the mountains were elevated athwart its course, and 
cut down its channel as fast as the land rose. Possibly it 
underwent a long process of adjustment to geological condi- 
tions, and experienced many vicissitudes due to changes of 
level, and has a far more complex history than the present 
features of its valley clearly indicate. At a late period it 
flowed far beyond the site of the great metropolis situated 
near its present junction with the sea, but a subsidence led 
to the submerging of its valley eighty miles eastward of 
Long Island, and transformed its upper course into an 
6 estuary as far as the city of Troy. The Hudson, in addition 
to its connection with American history and the wonderfully 
attractive scenes along its course, has a long and varied 
experience to relate. 



SOME CHARACTERISTICS OE AMERICAN RIVERS 261 

Appalacliian Rivers, — The Delaware, Susquehanna, Poto- 
mac, and James rise to the westward of the Appalachians 
and flow through the many separate ridges composing that 
wonderfully beautiful mountain system. They traverse the 
ridges in deep, narrow gorges, known as water-gaps, and 
enter wide-mouthed estuaries. Evidently the mountains 
did not exist at the time the courses of the rivers were 
established. The long ridges separating the picturesque 
and fruitful valleys in the Appalachian Mountains are 
level-topped, and rise in a large number of instances to the 
same general height. Fill the valleys to the level of the in- 
tervening uplands, and a plateau with an even surface would 
result. The restored plateau would slope south-eastward, 
and streams flowing down it would cross the tilted rock- 
layers composing it at right angles. The courses of the 
main streams, if once established on such a plane, would be 
maintained unless marked disturbances occurred, and would 
slowly deepen their channels and develop new branches. 
For tens of thousands of years the tireless streams would 
work at their task. The soft rocks would be removed with 
comparative ease, leaving the hard layer in bold relief. The 
result would be a deeply dissected plateau like that of 
eastern Pennsylvania or West Virginia. The filling in of 
this outline sketch of the origin of the bolder features so 
much admired by travellers over the Appalachian divisions 
of the Pennsylvania or the Baltimore and Ohio railroads, 
will give a picture of the geographical development of a 
large tract of rugged country embraced in the west-central 
portion of the Atlantic drainage slope. 

The rivers flowing eastward from the Appalachians cross 



262 RIVERS OF NORTH AMERICA 

a plateau composed of resistant rocks, and on its eastern 
border form cascades and rapids where they descend to the 
still lower coastal plain formed of incoherent strata. This 
line of cascades and rapids extends from near the Hudson 
to beyond the Savannah, and is frequently designated as 
the ** fall line." To the west of this line the swift-flowing 
rivers are shallow, while to the eastward, owing in part to a 
subsidence of the land and the consequent drowning of 
their lower courses, their currents become sluggish and the 
water deep. These geographical conditions resulting from 
a long series of changes, have had a marked influence on 
both the savage and civilised inhabitants of the Atlantic 
slope. The fall line, before the arrival of Europeans, was 
the site of numerous Indian villages. White men coming 
to America in ships could ascend some of the rivers to the 
fall line, and there found the head of navigation. In order 
to penetrate farther inland a new start had to be made. At 
these same localities water power was discovered, and man- 
ufactories established. For these and other reasons the fall 
line became a line of cities. If we draw a line on a moder- 
ately small-scaled map of the Atlantic States, through the 
sites of Trenton, Philadelphia, Baltimore, Washington, 
Richmond, Weldon, Raleigh, and Augusta, it will mark 
out a belt of the earth's crust which has experienced re- 
peated movements, and define with considerable accuracy 
the junction of the piedmont plateau with the coastal plain, 
and show the position of the fall line. 

Rivers of Glaciated Lands, — Tens of thousands of streams 
in the northern part of the Atlantic drainage slope and in 
the region to the north and west are broken by cascades 



Plate XV. 




Fig. a. The Columbia Looking West from White Salmon, Washington. 
(Photograph by J. H. Valentine.) 




Fig. B. The Hudson, from West Point, New York. 

(Photograph by W. H. Rau.) 

ILLUSTRATIONS OF DROWNED RIVER- VALLEYS. 



SOME CHARACTERISTICS OF AMERICAN RIVERS 263 

and rapids and drain many thousands of lakes. These 
features, which add a crowning charm to northern land- 
scapes, are almost entirely absent from the southern Appa- 
lachians and the Gulf drainage slope. Why this striking 
contrast ? Why should the streams from central Pennsyl- 
vania to Labrador plunge over precipices, or form foam- 
ing rapids, and the land they drain be studded with tarns, 
lakes, and great fresh-water seas ; while the streams in an 
equally elevated and fully as picturesque region, but with 
different details, at the south, flow in evenly sloping 
channels except on their extreme head-waters, through a 
land that. is completely drained and in which lakes similar 
to those at the north are absent ? The answer, thanks to 
Louis Agassiz and other students of glaciers, is that ice- 
sheets of vast extent gave a new surface to the land over 
the northern half of the continent, and to a great extent 
obliterated the valleys and stream channels made during a 
preceding period of mild climate and luxuriant vegetation. 
Southern Rivers, — What charming pictures of placid 
rivers flowing between wooded and flower-bedecked banks, 
softened and partially obscured perhaps by morning mists, 
enrich the memories of those who have travelled in the 
Carolinas, Georgia, and Alabama! Whence the fascination 
of these sleepy streams, flowing through flat-bottomed val- 
leys bordered by mildly roughened, plateau-like uplands ? 
What has subdued the broader features of the landscape in 
a region where every river bank reveals folded and con- 
torted rocks, similar to those in the neighbouring mountains? 
The geographer sees evidence at every turn that mountains 
once existed, but that they have been removed. A cycle 



264 RIVERS OF NORTH AMERICA 

of geographical history has run its course, and a new cycle 
has been initiated at a comparatively recent date. The 
land, once rugged and mountainous, has been carved away, 
with the exception of certain island-like remnants or mo- 
nadnocks, to a uniform level, — the horizon of the sea, — then 
moderately upraised and the surface of the gently tilted 
peneplain channelled by streams. 

Alluvial Rivers, — In the low-lying regions bordering the 
Gulf of Mexico, the traveller finds sluggish rivers flowing 
through broad valleys which are flooded each springtime, 
or in early summer, when the snow melts on the mountains 
to the north and west. Every stream is margined by natural 
embankments or levees, which, when not modified by human 
agency, are built higher during each succeeding flood. 
With the raising of the levees the valley bottoms are in- 
undated and a layer of fine rich soil deposited over them. 
Evidently the streams, instead of deepening their channels 
after the manner of the swift-flowing rivers of New Eng- 
land, are filling in previously formed depressions or making 
new lands along the Gulf coast. It is plain to be seen that 
the laying down and not the removal of material is in pro- 
gress over vast areas, and that rich lands are being formed^ 
on which rice and sugar-cane can be cultivated. No mount- 
ains or hills are in sight. The land is flat, without valleys, 
and except where fields have been cleared, is clothed with 
tangled vegetation. Cypress trees and white-trunked cotton- 
woods lean far over the yellow waters, their branches fes- 
tooned with trailing vines and pendant lichens. Much that 
is interesting concerning the manner in which streams fill in 
their valleys during certain stages in their histories, and the 



SOME CHARACTERISTICS OF AMERICAN RIVERS 265 

way in which new areas are reclaimed from the sea, may 
here be studied. The process of stream excavation, most 
active where the land is high, the gradients steep, and the 
waters swift, here finds its complement where the land is 
low, the gradients gentle, and the waters sluggish. The 
rivers, although large, are unable to bear along the sediment 
delivered to them in their swifter upper courses, and it is 
laid aside in flood-plains and deltas to rest until the facilities 
for transportation are more favourable. The southern por- 
tion of the Gulf drainage slope furnishes abundant illustra- 
tions of the fact that an important part of the work of 
streams consists in depositing material and the aggrading 
or filling in of their channels and valleys. The streams of 
New England and of the Gulf region, although presenting 
marked contrasts, are not essentially different, but have 
reached different stages in their life histories, and have felt 
the influence of diverse modifying conditions. 

The Mississippi, — Some of the characteristic features of 
the lower portion of this the greatest of all the rivers of 
North America have been included in the glance we have 
just given, which is all that space will allow, to the alluvial 
rivers of the Gulf coast. The great importance of the Mis- 
sissippi as a highway of commerce and of civilisation, the 
vast agricultural interests of its basin, and the numerous 
illustrations of the behaviour of streams under widely con- 
trasted conditions and in various stages of development fur- 
nished by it, tempt the student of American geography to 
visit its banks again and again, and to long to explore its 
entire extent with the searchlight of modern geographical 
methods. 



266 RIVERS OF NORTH AMERICA 

The source of the central trunk of the Mississippi is in 
Lake Itasca, but, as is well known, a great branch of the 
drainage tree, the Missouri, far overtops the summit of its 
central stem. To the geographer the true source of the 
Mississippi is at the as-yet-unknown fountainhead of the 
Missouri. The waters forming the Missouri are supplied in 
part by the hot springs and wonderful geysers of the Yellow- 
stone Park, and in part by snow banks and small glaciers in 
the more elevated valleys and amphitheatres of the Rocky 
Mountains, in Idaho and Montana. To a small extent the 
waters of the great river come from Canadian territory. 
The countless lakes of Minnesota and Wisconsin expand 
like leaves on the terminal twigs of the central trunk. The 
head-waters of the Ohio, the largest branch of the Missis- 
sippi which joins it from the east, rise on the western slope 
of the Appalachian uplift in West Virginia and Pennsyl- 
vania. A small portion of south-western New York is also 
included in the Ohio drainage basin. The distance in a 
straight line from the head-waters of the Ohio north-west- 
ward to the source of the Missouri is over eighteen hundred 
miles. From the mouth of the Mississippi along its general 
course to the continental divide, which limits its drainage 
basin on the north-west, is about twenty-five hundred miles, 
but including all of the windings of the river, the actual 
distance that the waters falling on the mountains of Mon- 
tana have to travel in order to reach the sea, is more than 
four thousand miles. The entire area drained by the 

Father of Waters " is about 1,240,000 square miles, or 
nearly one-third of the United States, exclusive of Alaska. 
I Additional statistics concerning the Mississippi, taken 



SOME CHARACTERISTICS OF AMERICAN RIVERS 267 

from the report of Humphreys and Abbot and from a recent 
report on the floods of that river, published by the Weather 
Bureau/ are here inserted: 

Average annual precipitation over the entire basin 29.8 inches. 

Annual discharge 785,190,000,000 cubic yards 

Ratio of rain-fall to discharge 0.25. 

Mean discharge per second 7 5, 000 cubic yards. 

That the Mississippi was in existence previous to the 
Glacial epoch is abundantly proven by the change that oc- 
curs in its valley when traced across the southern limit of 
the ice invasion, which intersects its course between the 
mouth of the Missouri and the Ohio. Southward of the 
glacial boundary it flows through a broad valley bounded by 
bluffs. The vast flood-plain, varying in width from five to 
eighty miles, lies from three to five hundred feet below the 
general level of the bordering uplands. The contour of 
the hard-rock bottom of the valley is but imperfectly known, 
but the records of wells and borings show that an ancient 
valley has been filled with alluvium to a depth of at least 
one or two hundred feet in its northern part and to an in- 
creasing depth southward. For about a thousand miles 
northward from the mouth of the river no hard rock appears 
in its bed, and cataracts are absent. The conspicuous bluffs 
of light-coloured clay-like material termed loess, bordering 
the flood-plain in many places, mark the borders of an inner 
valley, excavated in the material which formerly occupied 
the older and broader valley from side to side. The great 
outer valley, eroded in large part through nearly horizontal 
beds of limestone, is a record of the preglacial work of the 

' Park Morrill, Floods of the Mississippi. Washington, 1897. 



268 RIVERS OF NORTH AMERICA 

river; the inner valley, formed by the removal of soft inco- 
herent loess and sand, is of postglacial origin. 

North of the glacial boundary, the river, throughout much 
of its course, flows through a comparatively narrow, steep- 
sided valley, bordered by precipitous bluffs of hard rock, 
and in places the waters rush in foaming rapids or plunge 
over ledges of limestone. These narrow reaches have all 
the characteristics of young streams. At other times the 
valley broadens and its crumbling sides are crowned by 
towers and pinnacles of rock which bear every indication 
of long exposure to the air. These evidences of old age 
occur in what is known as the driftless area of Wisconsin 
and Minnesota, where an island-like area, measuring some 
ten thousand square miles, existed in the former ice-sheets. 

In some instances north of the glacial boundary, where 
the river flows through a narrow, rock-cut valley, deeper and 
broader channels adjacent to it but now filled with glacial 
debris, show that the river was turned from its preglacial 
course, when reborn after the ice invasion. In such in- 
stances, the greater size and depth of the old channels, in 
comparison with their modern representatives, bear evidence 
that the river, previous to the advent of the glaciers, had a 
greater length of time in which to carry on its appointed 
task, or else worked with greater energy than since the 
glaciers vanished. Many considerations tend to establish 
the former of these suggestions. The Mississippi was an 
aged stream before the great climatic change which per- 
mitted of the extension of glaciers from the north into its 
drainage basin. 

The main channel of the Ohio belongs to the extensive 



Plate XVI. 





Fig. a. An Aggraded Valley near Fort Wingate, New Mexico. 
Illustrative of the filling of valleys in arid regions ; the clififs are of Triassic sandstone. 



N^^ .v...,.>.' 




Fig. B. Water-Gaps Cut by the Potomac through 1 wo Kidges ot Hard Rock, 

near Harper's Ferry, W. Va. 

The point of view is on the Shenandoah peneplain ; the Potomac flows through a steep-sided 
trench about 225 feet deep, sunken in this peneplain. Loudoun Heights on the left and 
Maryland Heights on the right in the background. 



SOME CHARACTERISTICS OF AMERICAN RIVERS 269 

system of branching valleys formed by the sinking into the 
rocks of the preglacial Mississippi drainage, but its upper 
portion was deeply buried by ice during the height of the 
Glacial epoch. When the glaciers finally melted, the re- 
born streams found their channels blocked, and in many 
instances were turned from their former courses in the same 
manner as in the case of the streams of Wisconsin and 
Minnesota. 

When the Laurentian glacier retreated to the northward 
of the height-of-land now dividing the streams which feed 
the Mississippi from those flowing to Hudson Bay and the 
Great Lakes, several lakes came into existence, which were 
retained on their northern margins by the face of the re- 
treating glacier, and supplied southward-flowing streams. 
One of these glacier-dammed lakes, named in honour of 
Louis Agassiz, occupied what is now the valley of Red River 
and the Winnipeg basin, and supplied River Warren which 
flowed to the Mississippi. Another similar lake was found 
in the western part of the present drainage basin of Lake 
Superior, and had its outlet a few miles west of the site of 
the city of Duluth. Other lakes in this same category oc- 
cupied the southern part of the basin of Lake Michigan, and 
the western part of the Erie basin ; the former discharged in- 
to the Mississippi through the channel now being converted 
into a canal, just west of Chicago, and the latter flowed 
through the valley now occupied by the Wabash to the west 
of Fort Wayne, Indiana. 

During the time the Winnipeg and Laurentian basins 
were sending their surplus waters southward to the Mis- 
sissippi, not only was the run-off from the land probably 



270 RIVERS OF NORTH AMERICA 

greater than now on account of heavier rain-fall, but the 
vast snow- and ice-sheet which covered Canada was melting, 
and all the stream channels leading away from it were 
flooded. The volume of water contributed in these several 
ways and flowing through the Mississippi valley to the Gulf 
of Mexico must at all seasons have been far in excess of the 
greatest of the modern floods. It has been estimated by 
James E. Todd ^ that the Mississippi, during the geological 
springtime following the great winter known as the Glacial 
epoch, carried annually from eleven to twenty times the 
volume of water now reaching the Gulf of Mexico through 
the same channel in a single year. Whether the current of 
the rivefduring this great flood stage was vastly increased 
or not, depends on the former elevation of the land. These 
are reasons for believing that the region occupied by ice 
was depressed several hundred feet below its present posi- 
tion, and that the gradient of the Mississippi was much less 
than at present. The expanded rivers then resembled a 
great sea in which the loess and other similar deposits now 
occupying the greater Mississippi valley were spread out. 

The long preglacial history of the Mississippi, the many 
changes impressed by the glaciers directly on its tributaries 
from the Appalachians to the crest of the Rocky Mount- 
ains, and indirectly, owing to the vastly increased water- 
supply, on the character of the river all the way to the 
Gulf, make it a most instructive subject for study. An ad- 
ditional chapter in the life of the river is supplied by a 
modern submergence which allowed the sea to reach to the 
mouth of the Ohio, and of still later re-elevation. The ac- 

' Geological Survey of Missouri, i8g6, vol. x., p. 203. 



SOME CHARACTERISTICS OF AMERICAN RIVERS 27 1 



/ 



cidents, as they have been termed, in the normal develop- 
ment of streams, due to climatic changes and to movements 
in the earth's crust, thus find numerous and graphic 
illustration in the Mississippi Valley. Until the entire 
basin, however, has been examined as a unit, disregarding 
political boundaries, even a satisfactory outline of its entire 
geographical history cannot be written. 

Another phase of the wonderful story of the Mississippi 
deals with its influence on the early explorations of the in- 
trepid emissaries of Spain and France, and the final con- 
quests of its basin by the English, the vast agricultural 
importance of its rich lands, and its value as an avenue of 
commerce; but the influence of geographical history on 
human events pertains more properly to the domain of the 
historian, and cannot be treated even briefly at this time. 

Canyon Rivers. — The Colorado River, rising in the mount- 
ains of Colorado, Wyoming, and Utah, and flowing through 
a high and for the most part arid table-land, has carved in 
the solid rocks the most magnificent canyon that has yet 
been studied. The river, with its load of sand and mud, 
has been able to deepen its channel more rapidly than its 
bounding walls have been lowered by rain, rills, and other 
destructive agencies. The result is a steep-walled trench of 
such stupendous proportions that when its sides are seen 
from below they appear to be towering mountain ranges. 
The tributaries of the Colorado have also deepened their 
channels at approximately the same rate as the main stream 
has excavated its canyon. A great river with many 
branches has thus been sunken into the rocks, to the depth, 
over a vast area, of about one m.ile. Between the larger 



2/2 RIVERS OF NORTH AMERICA 

streams there are flat-topped table-lands, remnants of the 
great plateau across which the Colorado flowed in its infancy. 
The plateau has been slowly elevated, while the sand- 
charged streams, acting like saws, have dissected it. 
Throughout a region tens of thousands of square miles in 
area, every stream is in the bottom of a profound gorge of 
its own making. The remnants of the plateau between the 
canyons are waterless and desert. 

The Colorado throughout a large part of its course flows 
through a canyon that is from four to six thousand feet 
deep. The canyon walls are, for the most part, of horizon- 
tally bedded rocks of many tones and tints, and various 
degrees of hardness. Weathering has increased the variety 
of colours, and rendered them more brilliant than they are 
in the unchanged rocks. The rain and wind have sculptured 
the cliffs so as to give them the greatest imaginable variety 
of forms. The most vivid dream-pictures of gorgeous 
Oriental architecture fail to rival the temple- and cathedral- 
like forms, incrusted with harmoniously tinted decorations, 
which overshadow the Colorado for hundreds of miles. 




Fig. 23. Cross-Profile of the Canyon of the Colorado. (After W. H. Holmes.) 
Vertical and horizontal scale the same : one inch = 6375 feet. 

There is nothing of the same class in the whole world, so 
far as is known, to compare either in extent and height, in 
richness of colour, or in variety and intricacy of detail with 



SOME CHARACTERISTICS OF AMERICAN RIVERS 273 

the canyon walls in the southern portion of the Pacific 
drainage slope. The canyon of the Colorado is not an even- 
sided canal, but a great valley some fifteen or more miles 
across. In the bottom of this greater canyon, as may be 
seen from the accompanying illustration, Plate XVII., re- 
produced from a drawing by W. H. Holmes, one of the few 
artists who are true to nature, is sunken a much narrower 
and deeper inner canyon. The reader will be able to read 
in this picture some of the leading events in the geographi- 
cal history of the region of the Great Plateau. This outer 
canyon is clearly the record of a time when the land was 
some four thousand feet lower than now, and remained at 
that horizon for tens of thousands of years, while the river 
cut down its channel to baselevel and by lateral corrasion 
broadened its valley. The climate, at least near the close 
of this long period of uninterrupted work, was arid, as is 
shown by the precipitous character of the cliffs bordering 
the valley that was excavated. The river meandered in 
broad curves over the nearly level bottom of its valley, and 
when the land was again raised maintained its winding 
course, and owing to renewed energy, due to greater ve- 
locity on account of an increase in gradient, again began the 
task of corrading to baselevel. This new task imposed upon 
the river is not yet completed. The waters still flow swiftly, 
and vertical corrasion is still in excess of lateral wear and of 
weathering. The precipitous character of the cliffs border- 
ing the inner gorge, and the details in their sculpture, indi- 
cate that the climate has had its present characteristics 
throughout the greater part, and probably the whole, of the 
time since the energy of the river was renewed. 



274 RIVERS OF NORTH AMERICA 

The walls of the canyon of the Colorado are not even- 
surfaced precipices, but on either side of the river are but- 
tressed by outstanding ridges and retaining walls, with many 
lateral branches. Everywhere there are towers and pin- 
nacles, as well as innumerable alcoves and recesses. The 
main buttresses extend out for miles from the brink of the 
gorge and partially fill the profound chasm into which they 
descend. From within the purple depth of the canyon rise 
wondrous temple-like forms as gorgeous in colour and as 
rich in fretwork and arabesque as a Moorish palace. These 
shrines for Nature-worship, although minor features in the 
sublime panorama, tower as far above the shining stream 
flowing past their bases as the summit of Mount Washing- 
ton rises above the sea. 

As an illustration of the endless variety, both in form and 
colour, of architectural forms that Nature can sculpture from 
an upraised block of the earth's crust under certain condi- 
tions of climate and rock texture, the Colorado region is 
unrivalled. The student of earth-forms there finds many 
illustrations of the various phases that an upraised region 
passes through, in what may be termed its youth. In a re- 
view of the life histories of rivers, the Colorado furnishes an 
example of a stream yet young, so far as its advance in its 
appointed task is concerned, but one which, owing to un- 
usual opportunities, has surpassed many older but less 
favoured streams in the magnificence of the results accom- 
plished. The Colorado is not only a young stream, but has 
been termed a precocious youth. Its success, however, in 
producing wonderful scenery of a novel type, lies not so 
much in the amount of work performed, as in the fact that 



SOME CHARACTERISTICS OF AMERICAN RIVERS 275 

destructive agencies have spared the canyon walls as the 
stream entrenched itself. The climate is arid, and the 
wasting of the cliffs consequently retarded. 

Stupendous as are the results achieved by the Colorado, 
and wonderfully impressive as is the scenery along its 
course, the amount of work it has done — that is, the num- 
ber of cubic miles of rock removed — is small in compari- 
son with what has been accomplished in many regions of 
mild relief, where rivers in their old age flow sluggishly 
over a plain from which they have removed nearly every 
vestige of a former mountain range. The region of great 
plateaus drained by the Colorado will, under the action of 
the agencies now in operation, be reduced to such a plain, 
unless future upheaval again renews the youth of the river. 

Sierra Nevada Rivers. — The numerous bright, leaping 
rivers of the Sierra Nevadas, flowing through valleys three 
to four thousand feet deep and overshadowed by pine- 
clothed mountains, suggest many questions in reference 
especially to the influence of rock texture, changes in eleva- 
tion and glaciation on stream erosion, and on the origin and 
development of topographic forms. The valleys are nar- 
row, with usually little if any bottom-land. The rivers are 
swift and strong and carry along with ease all of the debris 
delivered to them by the bordering slopes and tributaries. 
Not only do they bear away all of the fine material that 
reaches them, but in times of high water roll along large 
boulders, and yet their capacity to transport is not satisfied, 
and they are clear, limpid, joyous streams during a large part 
of the year. The conditions are there the reverse of what is 
so manifest in the rivers of the Gulf States, where previously 



276 RIVERS OF NORTH AMERICA 

eroded valleys are being filled and broad areas of new 
land have been formed. In the Sierra Nevadas the streams 
are all at work at the task of deepening their channels; 
the stage in their lives when they will broaden their val- 
leys more rapidly than they deepen them has not been 
reached. The Tuolumne, King, Truckee, and many other 
rivers are not only young, but are still broken by cataracts 
and rapids, and in many instances are supplied in part by 
the overflow of lakes. These are plain evidences of youth. 
A little study shows one, however, that the tireless activity 
of the streams is largely due to a recent uplifting of the 
mountains, which has given them steeper slopes, and that 
the presence of waterfalls and lakes along their courses is in 
many instances due to the former existence of great snov/- 
fields and magnificent glaciers in all of the higher valleys. 
The energy with which the streams are working is thus seen 
to be due to a revival of activity, or a rejuvenation, rather 
than to actual youthfulness. 

The westward-flowing streams from the Sierra Nevadas 
experience a sudden change on leaving the mountains and 
entering the flat-bottomed valleys where they unite to form 
the San Joaquin and Sacramento. With loss of grade the 
waters flow less rapidly, and their burdens are dropped. 
Deposition and aggrading are then the rule instead of abra- 
sion and valley-deepening. Borings made in the bottom of 
the broad, nearly level-floored valley of California, show 
that a great depression between the Sierra Nevada and 
Coast mountains has been filled to a depth of many hun- 
dreds of feet. Much of this filling is due to the deposition 
of material swept out of the bordering mountains in order 



SOME CHARACTERISTICS OF AMERICAN RIVERS 277 

to form the gorges and canyons which give them much of 
their diversity and beauty. Conditions similar to those 
already noted on the Atlantic coast, where a subsidence of 
the land has transformed the river valleys into estuaries, 
are again manifest on the western border of the continent. 
The story of stream development and of changes in the re- 
lief of the land on the Pacific coast, due to the upheaval of 
the land, is supplemented by the effects of subsidence on 
the geography where land and ocean meet. The bay of 
San Francisco and its outlet through the Golden Gate show 
that valleys have been drowned, owing to a downward 
movement of the land. Surveys of the sea bottom adjacent 
to the present coast-line reveal the fact that former river 
courses may, in some instances, be traced over the conti- 
nental border now depressed beneath the Pacific, in the 
same manner that soundings have demonstrated a former 
seaward extension of the Hudson, St. Lawrence, and other 
streams of the Atlantic slope. 

It seems scarcely necessary to mention, so obvious is it, 
the intimate relation between geographical history and 
human activities, illustrated by the origin and marvellous 
growth of the metropolis of the Pacific coast on the border 
of a partially submerged river valley. The magnificent bay 
of San Francisco, one of the very finest harbours in the 
world, is a direct result of a long series of geographical 
changes. The subsidence which converted a portion of the 
valley of the Sacramento into an arm of the sea has had a 
direct and far-reaching influence not only on the lives of 
millions of people, but on the building of a nation. The 
future greatness of San Francisco, assumed by her com- 



278 RIVERS OF NORTH AMERICA 

manding geographical position, will make her an important 
factor in the spread of civilisation, not in America alone, but 
in the countries bordering the distant shores of the Pacific. 

*' Where Rolls the Oregon,'' — The Columbia and its main 
tributary, the Snake, rise in the Rocky Mountains, and flow 
across a region of small rain-fall, thus simulating some of the 
main conditions which have influenced the history of Colo- 
rado River. Snake River crosses a basaltic plateau and has 
excavated a magnificent canyon. Although inferior in the 
richness of its colouring and the profusion of details in its 
sculptured cliffs, it is comparable in many ways with the 
Grand Canyon of the Colorado. The walls of Snake River 
canyon are composed mainly of black basalt in horizontal 
layers, which assumes a great variety of cathedral-like and 
monumental forms on weathering. The architecture is 
locally varied where granite and schist are exposed in the 
lower portions of the profound gulf, but throughout hun- 
dreds of miles of great escarpments the dark basalt gives a 
sombre and even an oppressive gloom to the strange scenery. 
In its deepest portion, on the east flank of the Blue Mount- 
ains, the canyon is about four thousand feet deep and 
fifteen miles broad. As in the vast canyon carved by the 
Colorado, ridges and abutments from the main walls extend 
from either side far into the profound depths, and fill the 
depression so as to make it appear much narrower and 
deeper than it is in reality. 

The Columbia also flows in a canyon-like valley for much 
of its course after leaving the Rocky Mountains. In its 
wild passage through the Cascade Mountains, it is bordered 
by some of the most rugged river scenery to be found on 



SOME CHARACTERISTICS OF AMERICAN RIVERS 279 

the Pacific coast, but nowhere has it formed a canyon com- 
parable with that traversed by Snake River. 

The main subjects of interest to admirers of bold scenery 
as well as to the student of topographic forms and of stream 
development, presented by the vast region drained by the 
Columbia, centre in the relation of the drainage lines to the 
disturbances which have affected the rocks. In the Appa- 
lachians, as we have seen, many of the rivers flow across 
folded rocks and have cut water-gaps through the ridges; 
in the region drained by the Columbia the streams fre- 
quently cross rocky ridges formed by the upraised edges of 
tilted blocks of the earth's crust, and also give origin to 
water-gaps. In several instances sharp-crested walls of rock 
from a few hundred to two or three thousand feet high, 
have been upraised directly athwart the course of the 
Columbia or of its branches, but the rivers have not been 
turned aside. As the blocks were tilted and their edges 
slowly elevated, the rivers deepened their channels as fast 
as the land rose and thus maintained their right of way. In 
other instances, the waters were held in check for a time by 
the rising land, and caused lakes to form, but the barriers 
were slowly cut across by the out-flowing streams, and 
again, what may be termed gateways were opened through 
the ridges. The thickness of ancient lake sediments over 
broad areas in the region under discussion shows that earth- 
movements, similar to those which influenced the character of 
the present topography, have been long in progress and have 
produced profound geological as well as geographical changes. 

The tilting of blocks of the earth's crust in the region 
drained by the Columbia has not only produced ridges of 



28o RIVERS OF NORTH AMERICA 

the nature just referred to, but in certain instances the 
surface has been depressed, thus lessening the grade of the 
streams and causing them to deposit their loads and ag- 
grade their valleys. Broad areas have for this reason been 
transformed into nearly level alluvial plains. 

In the instructive Columbian region, and especially in 
that portion of central Washington known as the Big Bend 
country, where the climate is now arid and the rate of gen- 
eral waste from the surface due to atmospheric agencies 
small, lines of fracture and of moderate faulting have deter- 
mined the direction of the stream courses. The streams 
flow along lines of fracture and in some instances have ex- 
cavated canyons with one wall higher than the other. This 
is the only region in North America, so far as has been 
recognised, where the relation of streams to fractures in the 
earth's crust favours the once prevalent idea that valleys 
are due to breaks in the rocks, instead of resulting from the 
wearing action of streams. Even these minor examples, 
however, of the influence of fractures on drainage fail ta 
support the hypothesis referred to, since the breaks simply 
gave direction to the streams which subsequently excavated 
the valleys, instead of directly producing the depressions. 

Another feature of especial interest in the land of the 
** Oregon," illustrating the influence of climate on the lives 
of streams, is furnished by the Grand Coulee, a deep, steep- 
sided canyon which cuts across the plateau partially enclosed 
by the Big Bend of the Columbia. The Columbia once 
flowed through this great trench, having been turned from 
its present channel by the advance of a glacier from the 
mountains to the north. This dam of ice held the river in 



SOME CHARACTERISTICS OF AMERICAN RIVERS 28 1 

check and caused it to rise and form a long, narrow lake, 
the outlet of which was through the previously eroded 
canyon now known as the Grand Coulee. 

The Columbia flows directly through the Cascade Mount- 
ains nearly at right angles to their trend, in a wild and ex- 
ceedingly picturesque water-gap, the greatest of its class on 
the continent. The complete history of this most impres- 
sive topographical feature has not been made out, but the 
facts in hand suggest that the mountains, like the narrow, 
sharp-crested ridges to the eastward, are due to the upraising 
of a block or a series of blocks of the earth's crust, along a 
line or belt of faulting, and that the river deepened its 
channel as fast as the rocks rose. Possibly the elevation of 
the land was not uniform, but progressed by stages, and that 
when most rapid, the river, unable to maintain its grade, was 
ponded and lakes forrned. This explanation of the origin of 
the Dalles of the Columbia, and other gorges both above and 
below, must not be accepted too hastily, however, as the 
possibility of cross-fractures having given direction to the 
river and assisted it in its task has not been fully considered. 

Where the Columbia nears the ocean its waters lose their 
energy and expand into an estuary, in which the tides rise 
and fall for a distance of about one hundred and forty miles 
from the ocean.' From what has been said concerning the 

' Rev. Earl M. vVilbur of Portland, Oregon, has informed the writer by let- 
ter, on the authority of government engineers, " that the tide is felt in the 
Columbia as far as the Lower Cascades, which is, I believe, about 140 miles 
from the mouth of the river ; the extreme range there being al)out six inches. 

** In the Willamette, the Columbia's largest affluent, the tide is felt as far as 
Oregon City, where there are falls ; about 115 miles from the ocean. 

" The extreme range noted at Portland, about 100 miles from the ocean, is 
3.2 feet." 



282 RIVERS OF NORTH AMERICA 

drowning of stream-cut valleys both on the Atlantic and 
Pacific coasts, it will be readily seen that a modern subsi- 
dence has recently affected a great extent of the coastal 
region of the North-west, including south-eastern Alaska, 
and has allowed the sea to encroach on the land and trans- 
form the lower courses of many valleys into tideways. 

The extremely interesting problems presented by the 
region drained by the Columbia, and the magnificence and 
novelty of the scenery existing there, tempt me to detain 
the reader and consider more fully the origin of the mas- 
sive cliffs and of the terraces and landslides on their faces. 
There are yet other connections between the elements of 
scenery and the work of streams, and a wonderful story of 
the time when the land was again and again inundated by 
floods of molten lava, but as the object of this fireside recon- 
noissance is simply to indicate some of the more instructive 
features of the land which the study of streams assists in 
interpreting, we must hasten on. Our journey is northward. 

Rivers of the Far North- West. — Fraser River, fed by tens 
of thousands of twig-like branches on the western slope of 
the Cordilleran Mountain system, furnishes much informa- 
tion in reference to the manner in which a broad, high 
region becomes dissected by the streams flowing from it. 
Many of the branches of this splendid river have their 
sources in fine glaciers, high up among the glorious peaks 
of the Selkirks and neighbouring ranges, flow through wild, 
steep-sided gorges and valleys and unite to form a trunk 
stream which has sunken three or four thousand feet into 
the rocks. In following the steep bank of the Fraser, while 
making the transcontinental journey over the Canadian 



SOME CHARACTERISTICS OF AMERICAN RIVERS 283 

Pacific, one sees on every hand evidences of the work of 
streams. The topography is yet young, although deeply 
and boldly cut, but the valleys are narrow, barely wide 
enough to give the rushing, foaming waters a passageway. 

Throughout much of the trunk portion of the Fraser 
drainage-tree, the grade is sufficiently steep to insure a 
rapid current. The debris brought from glaciers, and fed 
by tributary rills and creeks, supplies the swiftly running 
waters with an abundance of tools with which to deepen 
their channels. Many conditions favour rapid work, and it 
is not surprising that the swift, debris-charged river has 
literally sawed a great mountain system into blocks, and is 
progressing rapidly with the task of removing the masses 
still remaining between its branches. The walls of its main 
canyon, although less precipitous than the bordering cliffs 
of the Colorado or the Snake, are wonderfully varied and 
picturesque. When seen from below they appear like deeply 
sculptured, forest-clothed mountain ranges. The river is 
yet young, but has accomplished a herculean task, and is 
still working with the energy of youth. As in the case of 
Snake River, the Fraser was interrupted in its work of cor- 
rasion during the Glacial epoch and its canyon deeply filled ; 
more recent corrasion has removed much of the alluvium, 
however, leaving well-marked terraces, as is illustrated on 
Plate VIII. Its canyon, although three or four thousand 
feet deep, has not yet reached the limit to which down- 
cutting is possible. Vertical corrasion is still in excess of 
lateral wear and weathering, and the great trench is V- 
shaped in cross-section instead of being broadly U-shaped, 
as will be the case in its mature life. Like the streams of 



284 RIVERS OF NORTH AMERICA 

the Sierra Nevadas, its energy is not all consumed in trans- 
porting the debris delivered to it, and for a large part of 
the year it rushes along as a foaming, roaring torrent carry- 
ing its load easily until it enters the coastal region, where 
a recent depression of the land causes a decrease in grade 
and a consequent loss of velocity. The river is shorter 
than formerly, for the reason that its trunk near the sea 
has been transformed into an estuary. The drainage-tree 
has been betrunked by subsidence and drowning. 

Glacier-Born Rivers, — North of the Fraser, and similar to 
it in the chief points of their histories, are the Stickine, 
Taku, Alsec, and other rivers which have their sources to 
the east and north of the mountains near the coast and flow 
through rugged and as yet but little-known lands to the sea. 
Probably all of the region drained by these rivers was ice- 
covered at a comparatively recent date, and thousands of 
glaciers still remain. Many are the lessons illustrated by the 
rugged landscapes of British Columbia and Alaska of the 
manner in which streams and glaciers modify topography, 
and the way that a subsidence of a deeply dissected land 
leads to the production of a ragged coast-line fringed with 
islands. This region includes the highest mountain and the 
largest glaciers in North America. The chief lesson that 
invites the geographer amid the ice-covered mountains near 
the coast is the influence of climate and of topography on 
the birth, growth, and decline of glaciers. This theme has 
been considered in a preceding volume.* 

The streams, many of them veritable rivers, flowing be- 
neath the glaciers, make highly interesting deposits of 

^ I. C. Russell, Glaciers of North America. Ginn & Co., Boston, 1897. 



SOME CHARACTERISTICS OE AMERICAN RIVERS 285 

gravel in the tunnels they occupy, and form alluvial cones 
and broad sand-plains after escaping from the ice. The 
study of these peculiar accumulations, still in process of 
formation, furnishes an explanation of many riddles in 
formerly glaciated lands. 

To the north of the narrow coastal portion of southern 
Alaska, where the surface waters are discharged directly to 
the Pacific, lies the great region drained principally by the 
Yukon, which forms the Bering drainage slope. The hydro- 
graphic basin of the Yukon embraces about 440,000 square 
miles, and the volume of the river, although as yet un- 
measured, is comparable with that of the Mississippi. 

The Yukon presents many of the characteristics of the 
rivers of more southern latitudes, and also possesses certain 
features peculiar to the streams of northern countries. 
Flowing, as it does in its upper course, from south to north, 
the wave of sunshine and warmth that sweeps from the 
equatorial to polar regions each recurring springtime 
reaches the lands drained by its head-waters and loosens 
the icy grasp of winter, while its lower portion is still ice- 
bound. The melting of the snow and ice and the spring 
rains at the south cause the streams to rise in floods, which 
advance laden with floating ice upon the still frozen 
country to the northward. Ice-dams are formed, and the 
streams expand and inundate the forest-covered valley 
bottoms. The rising waters finally break the ice-dams and 
rush on down the valleys carrying destruction in their paths. 
Trees are uprooted, or cut off by the floating ice as with a 
scythe. Vast quantities of earth and stones, enclosed in 
the ice that formed in shallow water, are borne along and 



286 RIVERS OF NORTH AMERICA 

deposited in part over the flood-plains of the streams when 
the ice melts. The energy with which the Yukon modifies 
its banks, on account especially of the ice-laden floods, is 
unrivalled by any more southern river. 

Another important variation in what may be considered 
as the normal action of streams arises, in the Alaskan 
region, from the constantly frozen condition of the soil. 
Throughout nearly the whole of the area drained by the 
Yukon the soil in the low lands is continually frozen. The 
winters are long and severe, the summers short and hot. 
The soil at a depth of a few inches beneath the usual cover- 
ing of moss, shrubs, and trees is perennially frozen. The 
thickness of the frozen subsoil is not known, but excavations 
twenty-five feet in depth have failed to penetrate it. Ice- 
clifl"s along the Kowak River, in North-western Alaska, re- 
veal a thickness of fully two hundred feet of dirt-stained ice 
beneath a thin layer of black mucky soil on which grasses 
and other vegetation thrive. From these and other observ- 
ations, and especially the records of certain borings made 
in a similar region in Siberia, it is safe to assume that the 
average thickness of the frozen layer in Alaska is probably 
in excess of one hundred feet and possibly two or three 
hundred feet or more in depth. These conditions have an 
important bearing on the work of streams. Frost renders 
otherwise loose and inadherent material as firm as solid 
rock. The action of flowing waters on the land is thus 
checked, and stream development as well as rock disintegra- 
tion and decay and general surface erosion greatly retarded. 

The climatic conditions in the region under discussion are 
such that the ground almost everywhere is covered with a 



SOME CHARACTERISTICS OE AMERICAN RIVERS 287 

dense growth of mosses and lichens, which make a living 
mat through which the surface waters percolate as through 
a layer of sponges, and are filtered of all matter in suspen- 
sion. Thus, again, the work of the streams is delayed, for 
the reason that sand and silt, which ordinarily constitute the 
principal tools with which flowing waters abrade the rocks, 
are removed. This process of filtering the water is illus- 
trated by the contrasts in the character of the tributaries of 
the Yukon which come to it from the north and from the 
south. Every stream, so far as is known, which joins the 
great river along its right bank is clear, although usually 
amber-coloured on account of the organic material contained 
in solution ; while the tributaries entering from the left, or, 
in general, the southern bank, are mostly turbid and heavily 
loaded with sediment, for the reason that they have their 
sources in glaciers. White River, one of the principal 
tributaries of the Yukon from the south, is charged with 
material in suspension not only because it is fed by melting 
glaciers, but for the reason that it flows through a region 
that is covered with fine volcanic dust, some of which is 
washed into the stream by every rain. The accidents to 
streams, as they have been termed, due to glacial and to 
volcanic agencies here find abundant illustration. 

The valley of the Yukon and of several of its important 
tributaries, particularly to the east of the Alaskan boundary, 
are marked by conspicuous terraces. A part of these are 
lake terraces, formed at a time when the waters were held in 
check by a lava dam, but other and equally conspicuous 
terraces were formed by the streams, and record changes in 
the altitude of the land, or the overloading of the waters 



288 RIVERS OF NORTH AMERICA 

with detritus during a time when glaciers near their sources 
were much more abundant and of far larger size than now. 
The head-branches of the Yukon drainage-tree rise in a 
country which was formerly covered with a continuous ice- 
sheet, but in its middle and lower courses evidence of for- 
mer ice occupation is wanting. Marked differences in the 
scenery beheld in journeying from one of these regions to 
the other have been noted by several travellers. 

Bering Sea, into which the Yukon empties, is shallow, 
at least in the portions bordering Alaska, and is without 
strong currents or high tides. The great river on entering 
the sea drops its heavy burden of silt, and has built up a 
delta comparable in extent with that of the Mississippi. 
The river divides into many branches, or sends off several 
distributaries in the delta portion of its course. The first 
of these diverging channels leaves the main river about a 
hundred miles from its mouth. The low, swampy area 
built by the stream is treeless, but clothed in summer with 
a dense growth of mosses, lichens, grasses, rushes, and a 
great variety of less conspicuous flowering plants. This 
luxuriant garden of brilliant flowers and luscious green 
fronds and leaves is but a veneer of verdure concealing a 
frozen morass. This is a portion of the vast treeless tract 
of perennially frozen ground known as the tundray which 
fringes the shores of Bering Sea and the Arctic Ocean. 
The several distributaries of the river flow through this new- 
made land in meandering courses, and enter the sea at 
various localities, over a breadth of seventy miles of coast. 

With the exception of the delta portion of the Yukon, its 
banks are fringed with spruce trees, cottonwoods, and 



SOME CHARACTERISTICS OF AMERICAN RIVERS 289 

willows. The annual floods and ice-gorges cause large 
numbers of trees to be swept into the river, and the swift 
■current at numerous localities cuts away the banks in such 
a way as to undermine the trees growing on them and cause 
them to fall into the waters with roots and branches at- 
tached. Great quantities of drift-wood are thus contributed 
to the river, as has already been described. The conserva- 
tive influence both of growing trees and of stranded drift- 
wood on the banks of the river are well marked and 
important, as are also the destructive tendencies of the 
same agencies. The trees on being uprooted tear away the 
banks, and on being stranded frequently deflect the current 
so as to cause it to cut away neighbouring shores and in- 
crease the number and extent of the windings of the river. 
Arctic Rivers. — Of the streams flowing down the Arctic 
drainage slope but little can be said, for the reason that 
no traveller especially interested in the study of modern 
geography has visited that region. The Mackenzie prob- 
ably illustrates the characteristics of a northward-flowing 
Arctic river even better than the Yukon. Much of the 
region it traverses is forested, and vast floods occur each 
spring when the thick ice of winter breaks up and is swept 
northward. The sudden changes that occur at the turn of 
the annual tide of temperature must be even grander than 
along the Yukon, but in this connection but little informa- 
tion is available. On entering the Arctic Ocean when the 
tides are low and currents produced by the winds mostly 
lacking, owing to the fact that the sea is covered with ice- 
floes throughout the year, the river deposits its sediment 

and is engaged in building a large delta. The three great 
19 



290 RIVERS OF NORTH AMERICA 

delta-making rivers of North America are the Mississippi^ 
Yukon, and Mackenzie. 

Rivers of the '' Great Lone La7td/* — On the Hudson Bay 
drainage slope there are tens of thousands of lakes, which, 
for the most part, occupy basins due in one way or another 
to the former occupation of the land by glacial ice. There 
are also many rivers, but the way in which they illustrate 
the principles of stream development has received but 
slight attention. The reports of explorers, especially those 
connected with the Geological and Natural History Survey 
of Canada, show that the drainage is immature. The 
streams have not cut down their channels so as to furnish 
direct and ready avenues of discharge for the surface waters. 
This is demonstrated especially by the countless lakes. The 
streams are not only young, having come into existence or 
having been rejuvenated since the last retreat of the glaciers, 
but have developed slowly on account of adverse circum- 
stances. Among the conditions that have retarded stream 
development may be noted the general low altitude of the 
land, and, consequently, gentle gradients of the stream 
channels and lack of energy in the flowing waters. The 
winter climate is severe, and the streams either ice-covered 
or frozen to their bottoms for several months each year. 
Snow protects the ground in winter. The subsoil, as in the 
Yukon basin, remains solidly frozen in many places even 
during the warm season. Erosion and the transportation 
of debris by the streams is thus limited to one half, or even 
less, of the year. Forests with undergrowths of mosses, 
lichens, and other plants shield the soil in summer from the 
beating of rain, and filter the percolating surface waters, 



SOME CHARACTERISTICS OF AMERICAN RIVERS 29I 

thus robbing them of the means of abrading the rocks over 
which they flow. There are no glaciers to supply the 
streams with sediment. The vegetation retards the gather- 
ing of the waters into rills, and equalises the flow of the 
streams in such a manner that the floods caused by melting 
snow have their energy diminished. The river banks are 
clothed with trees and shrubs, especially willows and alders, 
and their roots bind the soil and increase its ability to resist 
the attacks of the flowing waters. Drift timber lodged 
against the sides of the streams, especially fallen trees which 
still retain a hold on the land, also protect the banks. For 
these and still other reasons, the work of the streams has 
progressed slowly. They illustrate retarded stream develop- 
ment, or a long-continued youthful stage. In this respect 
they afford a marked contrast to the Colorado, where the 
opportunities for development have been unusually great. 
There is still another reason for the slow development of 
the streams flowing to Hudson Bay, although its full signifi- 
cance has not been determined. That is, the region toward 
which they flow is believed to be a rising area. The upward 
movement of the land is slow, although the rate is not 
known. An elevation of a very few inches a century would 
have a decided effect on the flow of the streams in a region 
of such mild relief. 

A glance at a map of North America suggests that a large 
number of islands into which the land is broken on the 
north-eastern border of the continent is due to a recent sub- 
sidence. There is geological evidence that over a vast area 
at the north, the land was depressed during the Glacial 
epoch and has since been slowly rising, but has not regained 



292 RIVERS OF NORTH AMERICA 

the position it held previous to the birth of the ice-sheets 
which once covered it. This re-elevation is thought to be 
still in progress, and should this conclusion be maintained 
it will furnish an additional reason for the present immature 
condition of the northward-flowing streams just referred to. 
Rivers Flowing to Fresh-Water Seas. — The St. Lawrence 
drainage slope, with its great lakes and magnificent rivers, 
affords numerous features of interest to the geographer be- 
sides its beautiful scenery. Soundings made in the Gulf of 
St. Lawrence, and even well to the eastward of the most 
easterly cape of Nova Scotia, have revealed the fact that 
the submerged channel of the St. Lawrence River may be 
traced on the floor of the ocean as far as the submarine es- 
carpment marking the true continental border. From the 
eastern extremity of this submerged channel through the 
Gulf of St. Lawrence and up the narrowing estuary to near 
Montreal, where the river at present meets tide-water, is 
more than a thousand miles. The Saguenay River, bordered 
by towering walls, occupies a canyon excavated by a branch 
of the Greater St. Lawrence. The same conditions are 
recorded in a less marked way by the Ottawa and other 
branches of the present river. We have here the most 
remarkable example of a drowned river-system that is 
known. The marginal portion of the continent with broad 
valley near the sea, leading inland to deep canyons, has 
been depressed in recent times so as to allow the sea to en- 
croach on the land. The valleys have become gulfs, bays, 
and estuaries, and the canyons narrow tideways ; highlands, 
that separated the former river valleys, when not completely 
submerged have been transformed into capes and head- 



SOME CHARACTERISTICS OF AMERICAN RIVERS 293 

lands, and in part surrounded by the sea so as to form 
islands. 

The influence of the geographical history of the St. Law- 
rence on the course of human events is even more strongly 
marked than in the case of similar changes along the Atlantic 
border to the southward. The estuary of the St. Lawrence 
furnished an easy passageway, reaching far inland, for early 
explorers, and its connection, by means of the unsubmerged 
portion of the ancient river, with the Great Lakes tempted 
the Jesuit missionaries to make bold canoe journeys into 
the very heart of the continent. This same route led to the 
establishment of missions and the planting of white settle- 
ments and trading stations on the shore of the Great Lakes 
and in the Mississippi valley before the passes in the Ap- 
palachians to the southward became known. In later years, 
a series of canals to facilitate navigation between the St. 
Lawrence estuary and the Great Lakes stimulated industry 
by bringing tens of thousands of square miles of forest 
and of rich agricultural land into communication with the 
markets of Europe. Far-reaching plans for establishing 
deep waterways along this general course of early canoe 
navigation are now being matured, and the influence of 
geographical conditions favourable to commerce will be felt 
still more potently in the future than they have been in the 
past.* 

To the south of the St. Lawrence estuary lies the charm- 
ing valley of Lake Champlain, which was excavated by a 
stream tributary to the Greater St. Lawrence when the land 

'I. C. Russell, "Geography of the Laurentian Basin," in Bulletin of the 
American Geographical Society ^ vol. xxx., pp. 226-254, 1898. 



294 RIVERS OF NORTH AMERICA 

stood higher than now. After acquiring about its present 
form, the Champlain valley was depressed and became an 
arm of the sea, which was inhabited by marine mollusks 
and frequented by whales. A tideway reaching southward 
connected with the submerged Hudson River valley, making 
New England an island. A partial re-elevation of the land 
caused the former gulf to be separated from the ocean, so 
as to form a saline lake. The rains and feeding streams 
furnished a supply of fresh water in excess of the amount 
lost by evaporation, and the salt waters were flooded out 
and the present stage in the history of the valley initiated. 
This marvellous transformation of a broad and well-de- 
veloped river valley to an arm of the sea, to a saline lake, 
and then to a fresh lake, in which the blue Adirondack Hills 
and the equally picturesque mountains of Vermont are re- 
flected, is one of the most instructive pages in the later 
geographical history of America. 

The story of the St. Lawrence valley and its tributary 
branches is supplemented by the no less instructive history 
of the basins of the Great Lakes, some account of which 
has been given in a companion to the present volume.* 

The student of river development and of the changes made 
by streams in the topography of the land, as he sails the 
Great Lakes and visits the thriving cities on their shores, 
sees records of a time when rivers flowed through the now 
water-filled basins, and for ages worked slowly at their ap- 
pointed task of deepening and widening their valleys. This 
great task, when far advanced, was more than once inter- 
rupted by the invasion of the entire St. Lawrence region by 

' I. C. Russell, Lakes of North Auicrica, Ginn & Co., Boston, 1895. 



SOME CHARACTERISTICS OF AMERICAN RIVERS 295 

glaciers from the north. Conditions now characteristic of 
central Greenland then prevailed where millions of homes 
are now situated, and where fruitful farms have replaced the 
desolation of ice-fields. When the great geological winter 
had passed, the former stream channels were clogged with 
debris, so as to retard the waters and cause them to choose 
new courses. Elevation and depression of the land over 
tens of thousands of square miles still further complicated 
the difficulties that the re-born streams had to contend with. 
The surface waters were held in check, and formed vast 
lakes in the partially obstructed and warped and deformed 
preglacial river valleys. 

When one marshals in fancy the changes that the St. 
Lawrence drainage slope has passed through from the time 
when it was more elevated than now and supplied a well- 
developed river-system, — that is, a river with many branches, 
which had cut down its channel nearly to sea-level, so as to 
have a low gradient for two thousand miles or more, and had 
broadened its valley so as to form a wide, open plain which 
extended far into the many tributary valleys, — through 
the marvellous changes incident to the glacial invasions, 
and the partial submergence beneath the sea, and the 
partial re-elevation of the drowned portion, the damming of 
the stream by glacial debris, and the changes due to warping 
of the earth's crust, the brief time that civilised man has 
been acquainted with the region becomes insignificant. In 
this hasty outline of a million or more years of geographical 
history, although seemingly crowded with important events, 
all of the changes experienced by the St. Lawrence drainage 
slope have not been included. There is evidence that the 



296 RIVERS OF NORTH AMERICA 

St. Lawrence basin has been in communication with the 
Mississippi River drainage. In the development of these 
two great river-systems, there has been a struggle for the 
possession of the land where they approach each other 
(analogous in some ways to the wars of the French and 
English for the possession of the same territory), which will 
be of interest to the reader who has followed the discussion of 
the backward cutting of drainage lines in a preceding- 
chapter. More than this, there are suggestions that the 
excavation of the basins of the Great Lakes was due in part 
to streams flowing southward instead of eastward, and that 
a change in direction was caused by movements in the 
earth's crust which are still in progress. The student of 
geography thus finds two chief lines of interest in the region 
under consideration, one dealing with the origin and history 
of the land forms, and the other with their bearing and in^ 
fluence on the current of human events. 

Niagara. — The lakes that first came into existence in the 
Laurentian basin during the final retreat of the glaciers 
were small and numerous. Many of them were short-lived, 
and were drained as the ice-dam retaining them withdrew 
north-eastward ; but some of them expanded with the retreat 
of the ice, and became vast inland seas, larger than any of 
their present representatives. At a late stage in the melt- 
ing of the glaciers, the basins now occupied by Lakes Erie 
and Ontario were occupied by a single great water-body. 
When the ice withdrew still more and the Mohawk valley was 
uncovered, a lower outlet became available and the waters 
escaped so as to lower the lake and cause it to be divided. 
The lake in the Erie basin overflowed across the dividing 



SOME CHARACTERISTICS OF AMERICAN RIVERS 297 

land to the Ontario basin, and Niagara River was born. 
An example of the manner in which a river may originate, 
not previously considered in this book, is thus furnished. 
The many happy tourists who have listened to the thunder 
of mighty Niagara and wandered along the brink of the 
gorge occupied by the waters in their mad course below the 
cataract, have many illustrations thrust upon their atten- 
tion of the manner in which streams modify the land. The 
cataract was once at the margins of the bold escarpment 
near Lewiston, and has slowly receded, leaving a great gorge 
as a record of its work. Unlike the magnificent canyon of 
the Colorado, or the almost equally remarkable example 
through which Snake River flows, the steep-walled gorge of 
the Niagara has not been worn out by the flow of silt- and 
sand-laden waters, but as already described in discussing the 
migration of waterfalls, illustrates another process by which 
the land may be deeply trenched. The waters of Niagara 
come directly from a great lake in which they have left all 
of the sediment they may once have held in suspension, and 
are clear. The deep tourmaline-green of the plunging cata- 
ract is never clouded. Clear streams, as we are aware, have 
but little power to deepen their channels, as all the debris 
available for transportation is soon removed, and the chem- 
ical action of the waters in dissolving the rock over which 
they flow is so slow that only in exceptional instances are 
they able to deepen their channels more rapidly than the 
adjacent surface is lowered by weathering. How, then, 
has the canyon below the Falls of Niagara been excavated ? 
The energy of Niagara River available for canyon cutting 
is concentrated at the base of the cataract. The river came 



298 RIVERS OF NORTH AMERICA 

into existence with a cataract, which was even grander when 
it first leaped from the crest of the escarpment at Lewiston 
than it has been since the first white man kneeled in its 
awful presence. Owing to the southward dip of the rocks, 
the height of the fall has been continually decreasing and 
will continue to decrease until it has receded to the lake 
from which the river flows, or the westward tilting of the 
land, known to be in progress, diverts its waters. The sur- 
face rock along Niagara River is a hard limestone about 
eighty feet thick; beneath this there are soft shales, much 
broken by joints and easy of removal. The dash of the 
spray, the grinding of ice-blocks, and, to some extent, -the 
freezing of absorbed water, leads to the removal of the shale 
so as to leave the limestone above projecting. From tim^e 
to time masses of the limestone break off and fall into the 
pool below. When the plunging waters have sufficient 
power to move these blocks, they are dashed against the 
cliff and act as millstones in deepening and widening the 
basin below. When the descending waters do not have 
sufficient power to sweep about the blocks of stone, as in 
the case of the American Fall, they accumulate and form a 
talus slope which protects the cliffs, and retard their seces- 
sion. This explanation, first offered by Gilbert, furnishes 
a reason for the marked differences between the American 
and Canadian portions of the cataract. Many other inter- 
esting and instructive features of Niagara are described and 
explained in the monograph just referred to. 

Retrospect. — We have made this rapid review of the prin- 
cipal drainage slopes of North America for the purpose of 
refreshing our memories concerning the more pronounced 



SOME CHARACTERISTICS OF AMERICAN RIVERS 299 

geographical features of the continent due to erosion. 
Another aim has been to suggest questions which the student 
of geography will find pleasure in answering. I fear, how- 
ever, our hasty journey has in some respects left an errone- 
ous impression on the reader's mind, for the reason that in 
considering each drainage slope only its more pronounced 
features have claimed attention. If each river appears 
to be principally and essentially different from all other 
streams, and the several drainage systems present an infinite 
variety of disconnected facts, modifications and corrections 
of such ideas are necessary. A more detailed study of the 
behaviour of streams will show that law and order prevail. 
From the purling rill to the majestic river, where at first, 
perhaps, endless variety appears, the flow of water and the 
changes it produces in the relief of the land are governed by 
inflexible laws. The streams one and all are engaged in a 
definite and well-circumscribed task, which leads to an 
orderly succession of topographic forms. When all of the 
modifying conditions are taken into account, the successive 
changes experienced by a given land area, from the time of 
its upraising above the sea, to the time when it is worn 
down nearly to sea-level once more, are seen to be as much 
in obedience to law as the seasonal changes in a landscape 
or the development of an individual man from childhood 
to old age. 

Although the origin of topographic forms and the many 
metamorphoses they undergo, claim the special attention of 
students of geography, the fact should be borne in mind 
that the knowledge thus gained is but the basis of a more 
profound study, — the relation of man to nature. The in- 



300 RIVERS OF NORTH AMERICA 

fluence of the earth's history on human history, although in 
many instances not fully realised, is an underlying and ever- 
present source of interest and enjoyment to the geologist 
and geographer. 

The brief review of some of the characteristics of rivers 
given in this chapter, it is hoped, will stimulate a desire, es- 
pecially in American students, to know more of the many 
and varied charms of their native land. 



CHAPTER IX 

THE LIFE HISTORY OF A RIVER 

AN application of the laws governing the behaviour of 
streams in interpreting the origin and history of topo- 
graphic forms can be made in almost any land area on the 
earth. In order to group in a single panorama, however, 
all of the various phases which a river passes through from 
its birth and youth to its old age and death, the conditions 
presented by many streams in various stages of growth and 
decline have to be combined, for the reason that the life of 
a man is too brief to enable him to observe more than a few 
minor changes in the history of a single river. But know- 
ing the laws which govern stream development, one can 
easily picture in his mind the leading events in the life of 
a majestic river whose murmurs we may be pardoned for 
fancying make audible the memoir of a million years. 

In order to sketch in outline the life history of an ideal 
river, let the reader imagine that the floor of the sea in 
temperate latitudes over an area of a hundred square miles 
has been upraised so as to form an island ; and trace the 
changes which will follow as the rain-water falls on its sur- 
face, and gathers into rills which unite one with another until 

301 



302 RIVERS OF NORTH AMERICA 

a series of rivers conducts the contributions from the clouds 
down their shining courses to the sea. 

The surface of our imaginary island is mildly irregular. 
In the central portion it has an elevation of a thousand feet, 
and slopes gradually but somewhat irregularly in all direc- 
tions to the sea. In places the waters gather in hollows 
and form lakes. These consequent lakes are soon filled, or 
their overflowing waters cut notches in the rims of their 
basins, and they are drained. The first streams that are 
born of the showers, like the children of men, have their 
courses marked out for them in early life, or, in more prosaic 
language, are consequent streams. Later in life they carve 
out their own fortunes and influence their surroundings. 

In these fireside fancies we assume the point of \'iew 
granted the novelist, to whom time and distance offer no 
limitation. A Scott or a Hawthorne tells us with confidence 
the most secret thoughts of a prisoner in his cell a century 
before they themselves were born. We accept the illusion 
so long as the laws governing human nature are not violated. 
Why should a similar privilege be denied the geographer ? 
Let us, then, trace the changes that our island will undergo 
in obedience to the laws of the inanimate world,' accepting 
the remark of Lamarck applied to the development of species, 
that ** time is nothing." The only supernatural condition 
which I will ask the reader to accept, is that the promontory 
on which we keep our vigil remains unchanged. 

Looking across the shimmering sea of fancy, v/e see the 
new-born consequent streams appearing like shining threads 
of silver when the skies are clear, but when the rain descends 
in torrents and the soil is loosened and disturbed they be- 



THE LIFE HISTORY OF A RIVER 303 

come yellow with sediment. Already changes are in pro- 
gress. The streams charged with silt and sand are corrading 
their channels. The lines thus produced are delicate at first, 
but soon become more and more deeply engraved. These 
infant streams have their sources not at the summits of the 
island, but in general midway down its sides. The deep- 
ening of the channels leading from the higher portions of 
the island to the sea makes the waters flowing down them 
master streams. As they sink deeper and deeper into the 
rocks, lateral streams are developed on the original inter- 
stream areas. These branches become swifter as the main 
streams deepen their channels, and in turn develop branches 
to which they themselves are masters. The secondary and 
tertiary branches cannot excavate below the level of the 
stream to which they contribute their waters, but the down- 
cutting at their mouths may keep pace with the lowering of 
the receiving channel. This process of throwing out new 
branches and the growth of each branch by terminal bud- 
ding, as it were, soon lead to the complete drainage of the 
land. Water falling on any portion of the island finds a 
system of channels, delicately adjusted in size in accord with 
the part they have to play, v/hich lead it back to the sea. 

Our island, we will assume for simplicity, is composed of 
nearly horizontally bedded rock of various degrees of hard- 
ness. The influence of the dip of the beds beneath the 
original surface, discussed in a previous page in connection 
with the adjustment of subsequent streams and the develop- 
ment of drainage systems under the conditions there de- 
scribed, need not be repeated. 

The headward growth of the feeding rills and brooks of 



304 RIVERS OF NORTH AMERICA 

the main consequent streams brings them into rivalry with 
each other. The boundaries between opposite-flowing 
streams in the central portion of the island become more 
and more sharply defined, and the positions of the divides 
can be easily traced. From these divides the descent into 
the valleys on either side is steep. Hard layers in the 
nearly horizontal beds cause many cascades. The young, 
joyous streams fill the air with laughter. The cascades came 
into existence low down the course of the streams and 
gradually retreated toward the centre of the uplift, leaving 
shadowy gorges as records of their migrations. It is only 
when the streams in their lower courses have deepened 
their channels nearly to sea-level that they cease to be 
w^hitened by cataracts and rapids. 

As we watch the growth of the streams we note that they 
deepen their channels most rapidly not at their mouths, nor 
at their sources, but at some locality between, which differs 
in its relative position, in various instances, with the size of 
the stream. The rate at which the streams we are observ- 
ing entrench themselves depends, as we know, on their 
volume, the declivity of their channels, and on the amount 
and character of the loads they carry. It is the resultant 
of these main conditions, rock texture being essentially the 
same throughout their courses, which determines at what 
locality conspicuous changes will first appear. Near their 
sources the grade is steep, but the waters are divided, flow- 
ing in numerous channels, and the work they are enabled to 
accomplish is not so great as farther down the slopes where 
many tributaries have united their energies. We need not 
again consider the various elements of a stream energy, 



THE LIFE HISTORY OF A RIVER 305 

but from the fact that the channels through which they flow 
become conspicuously deeper midway up the slopes of the 
island, it is evident the most rapid corrasion is there taking 
place. 

Below the locality of most rapid corrasion the slope is less 
precipitous, and although the volume of water is greater, 
the rate at which the streams corrade decreases all the way 
to the sea. The amount of rock that has to be removed 
in order to admit of the sinking of the channels to base- 
level, however, is less and less the nearer they approach the 
coast-line. The streams near their mouths are thus enabled 
to reach the downward limit of their task sooner than at 
any locality higher up their courses, in spite of the fact that 
they there work more slowly than elsewhere, except perhaps 
at their extreme head-waters. Whatever the conditions, it 
is evident that any portion of a stream at a distance from 
its mouth cannot be lowered to baselevel more quickly than 
the portion nearer the sea, unless possibly by solution, as the 
material removed would in such a case have to be carried 
up instead of down a gradient. 

As we watch the changes in progress, we note that after 
the first adjustment to inherited conditions is made, all of 
the material removed where the stream beds are steep, is 
not carried directly to the sea. At first, perhaps, the slopes 
were such that the debris contributed to the master streams 
could be carried all the way to their mouths, but such an 
adjustment of gradient to load at the start would be of the 
nature of chance. The probabilities of a stream's inheriting 
a gradient perfectly adapted to its needs are almost in- 
finitely small. Throughout the life of a stream, even 



306 RIVERS OF NORTH AMERICA 

though external conditions remain unchanged, there is a con- 
stant process of adjustment of gradient to suit the changing 
conditions due to corrasion and sedimentation in various por- 
tions of its channel, and also to variations in volume and load. 

This process of adjusting the gradient of a stream channel 
in its several parts to particular conditions of volume and 
load, is so delicate that no two of the streams we are watching 
will carry on their work in precisely the same way. In 
most instances, the debris removed midway- down the course 
of a stream, where corrasion is most active, will in part be 
deposited lower down and aggrading begin. Other streams 
will deepen their channels at their mouths to baselevel, and 
then begin to broaden their valleys and spread out flood- 
plains. As the streams grow older, the portions of their 
courses where corrasion is in progress will slowly recede up 
stream and be followed, at least for a time, by an extension 
in the same direction of the increasing flood-plains. 

Clouds gather about our island from time to time, and it 
experiences all the vicissitudes of climate entailed by the 
position it occupies on the earth's surface. Vegetation 
springs into existence, and the land is clothed with grasses 
and flowers, or deeply shadowed by forests. The length of 
our vigil" is so great that possibly the character of the flora 
undergoes many variations owing to climatic changes. Al- 
though these modifications in conditions vary the lives of 
the streams, they do not stop their w^ork. 

When the streams have deepened their channel where 
they approach the sea nearly or quite to baselevel, vertical 
corrasion ceases and is followed by aggrading, while lateral 
corrasion continues. 



THE LIFE HISTORY OF A RIVER 307 

If we select one of the several larger consequent streams 
for special study, we find during the earlier stages of its life, 
that debris is continually being supplied by its swift upper 
branches in excess of the amount the sluggish current in its 
main trunk can carry away. In consequence, the flood- 
plain downstream, from the localities where corrasion is in 
progress, is built higher and higher. During high-water 
stages accompanying heavy rains, the stream meanders at 
will over the flat bottom it has given to its valley, and 
divides into many branches. The position of the stream is 
unstable, for the reason that abundant deposition of debris 
raises its bottom and borders, thus elevating it above the 
adjacent areas. When floods occur, the stream breaks 
through its levees and chooses a new channel, which is 
built upon as before, and the process repeated. 

At this stage in its history the stream to which we have 
directed special attention will have many high-grade 
branches, in which corrasion is in active progress, and a 
low-grade trunk portion where debris is being deposited. 
When the stream is corrading, the valleys or gorges are 
steep-sided and present V-shaped cross-profiles, but below 
the region of waste where a flood-plain is being formed, the 
valley is wide, essentially flat-bottomed, and has flaring 
sides. It is to be noted also that in the alluvial-filled valley 
there are no terraces. During this still youthful stage the 
trunk stream has many curves, and divides into several 
branches during floods, so as to enclose low% sandy islands. 
These changes in the position of the main channel are 
ifregular, and frequently rapid. 

The grading up of the main valley in the manner just 



308 RIVERS OF NORTH AMERICA 

noted, was necessitated by the high grade of its tribu- 
taries. The aim, we may say, was to make an approxima- 
tion to the easiest attainable pathway for the debris on its 
journey to the sea. Theoretically, such a pathway would 
be what is known as the '' curve of quickest descent. '' But 
the down-cutting of the stream channels in their upper 
courses continually changes the conditions, and a continual 
process of adjustment in the lower and flatter portions of 
the curve is thus necessitated. As the grade of the tribu- 
tary streams is thus reduced, the region of previous aggrad- 
ing must also be modified. Hence there comes a time 
when the stream in its trunk portion begins to excavate a 
channel through its previously formed flood-plain. 

In this stage of adjustment, centuries being numbered as 
hours, we see the river writhing in its course through its 
more level tract; now cutting away the rocks on one side 
of its valley and then swinging bodily across its flood-plain 
and attacking the opposite bluff. Each of these migrations 
is accompanied by a multitude of minor contortions. Its 
course is always serpentine. On each of the minor bends 
we note that the immediate river bank is steepest on the 
concave side of the curve made by the stream, while on the 
opposite or convex side the bank slopes gently upward to 
the level of the flood-plain. The stream is plainly at work 
in removing material from the concave, and making additions 
to the convex, side of each curve. It soon becomes appar- 
ent that the stream is working over the material forming at 
least the surface portion of its flood-plain. With each dis- 
turbance of the detritus previously deposited, it is carried 
farther on its way to the sea, but each journey is short for 



THE LIFE HISTORY OF A RIVER 309 

all but the very finest material, which is taken in suspen- 
sion and may be borne to the sea with but short rests on 
the bed of the stream. This process, as we know, leads 
to an assorting of the debris of which the flood-plain is 
formed : the coarsest portions are dropped first, and on them 
finer and finer sediment is laid down, the last addition to 
the plain being the finest of all. With each period of rest 
in the flood-plain, the debris undergoes chemical changes, 
and is more or less affected by frost and variations of tem- 
perature, which softens and weakens it so as to favour more 
rapid wear when next it is removed. 

The increase in the curvature of the minor bends of the 
meandering stream leads from time to time to the cutting 
through of the neck of land between two adjacent curves, 
and the straightening of the contorted channel. A shorter 
course is thus made for the waters, which is followed by re- 
adjustment of grade both up and down stream, and the 
former abrupt curve is left as a bayou, the entrance and exit to 
which soon become closed and an '* ox-bow lake ** is formed. 

A single migration of the river across its flood-plain re- 
quires thousands of years. But during this time its channel 
sinks deeper and deeper into the previously deposited debris, 
and an entire migration from one side of the valley to the 
other and back again is not always completed before a 
second swing is begun. The portion of the flood-plain not 
worked over during one of these incomplete migrations re- 
mains as a terrace. With each migration a flood-plain is 
spread out, and wherever a flood-plain formed during the 
preceding migration is not completely worked over, a terrace 
'is left as a record of the unfinished task. 



3IO RIVERS OF NORTH AMERICA 

The branches of the river have now cut down their chan- 
nels so as to have a comparatively low grade, except at their 
extreme head-waters, and the trunk of the drainage-tree is 
contorted and bordered by alluvial terraces. 

While the changes outlined above have been in progress, 
the fine debris carried by the river to its mouth has been 
deposited so as to form a low-grade delta, which makes an 
addition to the land, thereby increasing the distance to 
which the river has to carry its load before it can finally lay 
it aside. Even the depositing of debris in a delta, however, 
can scarcely terminate the influence of the river upon it, as 
the surface of the delta is but a continuation of the flood- 
plain, and future adjustments to ever-changing conditions 
may necessitate its removal and re-deposition in a later 
seaward extension of the land. 

/ The growth of the delta, by increasing the length of the 
river, necessitates that its gradient throughout the portion 
where flood-plains occur should be raised in order to facili- 
tate the transportation of fresh debris over it. A continual 
check is thus placed on the process of down-cutting in the 
alluvial filling of the valley, necessitated by the constantly 
decreasing grade of the corrading tributaries. This con- 
tinual re-adjustment of the gradients throughout a drainage 
system, be it a meadow brook or the Mississippi, with ever- 
changing conditions of corrasion and sedimentation, is one 
of thousands of illustrations of the harmony of Nature. It 
was the result of this process of continual adjustment which 
forcibly impressed Hutton and Play fair, nearly a century 
ago, as is shown by the passage quoted on a prefatory page 
of this book. 



THE LIFE HISTORY OF A RIVER 31 1 

During the centuries that have passed while we have been 
considering the work of the streams flowing from the island 
before us, wonderful changes have taken place in the topo- 
graphy of its surface. The sinking of the stream channels, 
but partially counteracted by aggrading, has left ridges be- 
tween the drainage lines. The land has been roughened by 
the cutting of valleys and canyons. The degree of this 
roughening depends mainly on the altitude of the land, the 
stage of development reached by the streams in various por- 
tions of their courses, and the amount the land has been 
lowered by general erosion. The streams are yet young, 
but, as already noted, have advanced farthest with their 
task of removing the rocks down to sea-level in their sea- 
ward portions. In the regions of low relief, near the sea, 
the streams have already passed the youthful stage, and are 
subduing the landscape not only by lateral corrasion but by 
•deposition. In this portion of the island, also, vertical cor- 
rasion having ceased, the tendency of weathering to reduce 
the interstream areas to the level of the adjacent valleys is 
no longer counteracted. In the higher portions of the island 
the divides between the main streams and between neigh- 
bouring branches of the same trunk drainage-line, are sharp- 
crested ridges. There is a wonderful and beautiful system 
displayed by these ridges. They are not level-topped, but 
marked by peaks and downward-curving saddles. On each 
ridge, whether a main divide or the crest of a branching spur, 
where two streams head against each other, there is a sag 
or saddle in its crest-line, and, where lateral spurs or sec- 
ondary or tertiary ridges join the main divides or a second- 
ary ridge, there are peaks or rounded knobs. The upland 



312 RIVERS OF NORTH AMERICA 

with its multitude of crest-lines and many peaks resembles a 
vast tent supported by many poles. The place of each sup- 
port is marked by a peak, rounded and given a convex 
curvature by weathering, and between the peaks the tent- 
cloth descends in gracefully curving folds. 

The branches of the stream are most numerous where the 
original slopes were steep ; the interstream areas are there 
narrow and sharp crested ; farther toward the sea, where 
the slopes are more gentle, the interstream areas are broader 
and flatter. There are two principal reasons for these dif- 
ferences. On the steeper slopes the run-off is greater in 
proportion to the total rain-fall than on lower slopes, because 
the less the slope the longer the waters are retained on the 
surface and the greater the loss from evaporation and per- 
colation. A more important result, perhaps, is that on 
steep slopes corrasion is rapid in proportion to weathering 
and general degradation, while with progressively decreas- 
ing declivity weathering more and more nearly keeps pace 
with corrasion. 

The ridges between adjacent streams not only lose their 
even crests as determined by the original slope of the land, 
with the progress of stream sculpturing, but if we look 
down on them from a point vertically above, it is apparent 
that they are sinuous lines. The eating back into the up- 
lands of opposite-flowing streams has not been uniform, but 
the divides have been pushed one way or another according 
to the rate at which the rival streams have been enabled to 
progress with their work. The divides migrate toward the 
weaker streams. The ridges forming both the main and 
secondary divides are sharp and steep-sided where opposing 



THE LIFE HISTORY OF A RIVER 3 13 

streams have eaten back the farthest, and are broader and 
have more gentle slopes where diverging ridges meet. The 
hills as well as the valleys are ever changing and record 
the workings of laws which produce infinite variety with 
the constant preservation of harmony and beauty. 

At a more advanced stage in the history of our island, the 
peaks standing at the junction of lateral ridges become 
more and more prominent as the saddles between them are 
deepened. Those well down the general slope in time be- 
come so far isolated that they stand as individual eminences, 
and their connection with the central system is at the first 
glance not apparent. 

The island has now reached its greatest topographic diver- 
sity, and, unless the orderly progression is disturbed, as by 
renewed elevation, for example, future changes will be in 
the direction of subduing its relief and smoothing out its 
contours. 

The process of broadening the valleys in their lower courses 
is extended farther and farther toward their sources. Broad 
flood-plains in time reach well into the central group of hills. 

In the uplands, valley-deepening continues, but the ratio 
of corrasion to general erosion becomes less and less with de- 
crease in the gradients of the streams. With decrease in eleva- 
tion, general erosion or degradation also diminishes, and both 
corrasion and degradation cease when baselevel is reached. 

As the gradient of the streams in their upper courses 
diminishes, the loads they carry to lower tracts also become 
less, and the streams are enabled to cut still deeper into 
their previously formed flood-plains. With the advance of 
old age the gradients of the streams become less and less 



314 RIVERS OF NORTH AMERICA 

throughout their lengths, but are always steeper near their 
sources than at any locaHty farther downstream. 

Our island now consists of two portions in reference to 
topographic development : a central region of prominent 
peaks and ridges, and a surrounding baselevel plain, covered 
with a sheet of stream-deposited debris. 

If at this stage a comparatively sudden elevation of the 
whole island occurs, which carries it up, we will assume, one 
hundred feet, conspicuous changes will follow. The gradi- 
ents of the streams at their mouths will be increased. Some 
of them may plunge into the sea over escarpments, thus 
forming cascades, which will recede up stream, leaving 
sharply cut ravines. The gradients of the streams will be 
re-adjusted to meet the requirements of the changed condi- 
tions. Their revived energy will enable them to corrade 
throughout their length and to quickly deepen their chan- 
nels, especially in the previously alluvial-filled valleys. This 
rapid deepening will lead to the abandoning of portions of 
previously occupied flood-plains, and terraces will appear. 
The streams flowing through their new steep-sided channels 
will at first retain the positions they chanced to have at the 
time of the uplift, and will be sinuous, but their increased 
energy will tend to straighten their courses. The terraces 
left by the sinking of the streams will be broad in the coastal 
plain and become narrower and narrower farther inland. 
At the same time the vertical elevation of each terrace above 
the adjacent portion of the new stream channel will decrease 
from the sea margin inland to where the stream it borders 
ceases to be a depositing stream and the tract where corra- 
sion is in progress is reached. 



THE LIFE HISTORY OF A RIVER 315 

At this stage in the history of the island, its marginal 
tract is an upraised peneplain, surrounding a group of hills, 
or monadnocks. 

The streams again broaden their valleys in their lower 
courses, the broadening terraces are removed, and this 
change sweeps inland until perhaps all records of the once 
conspicuous peneplain disappear. 

While we are pondering on the changes produced by 
slowly acting forces when the time limit is ample, our island, 
in obedience to unseen causes deep within the earth, is 
depressed two hundred feet. The sea encroaches on the 
land, and, extending far up the valleys, converts them into 
estuaries. The highlands between the valleys become capes, 
and possibly some of the outer members of the central 
group of hills are entirely surrounded by water and are 
transformed into islands. The coast-line, which previous to 
the subsidence, was conspicuously regular and formed long, 
sweeping curves, is now markedly irregular. The sea-water, 
extending far up the deeper and more thoroughly developed 
valley, forms long, narrow estuaries or downward valleys^ 
of the type of the Hudson estuary. In some instances the 
drowning of the valleys has extended above where the lower 
branches of the former stream joined the main trunk, and 
the lower courses of the tributary valleys are now bays 
on the side of the main estuary. These are estuaries of 
the Chesapeake type. In such instances the trunks of 
the former river systems have disappeared, and only 
their dismembered branches remain. The drainage-trees 
have been betrunked by subsidence. At this stage of 
greatest geographical diversity, so far as the relations of sea 



3l6 RIVERS OF NORTH AMERICA 

and land are involved, the shores of the island are indented 
by numerous bays, many of them having flat alluvial lands 
at their heads. 

In fancy we have clothed our island with a varied flora. 
The picture presents a pleasing grouping of swelling hills 
with rounded summits and gradually sweeping sides, wide 
valleys with gently sloping borders, separated in part by 
broad but yet well-drained plains. These various forms are 
modulated and their details concealed beneath a living man- 
tle of vegetation. Seasonal changes, recurring like the fig- 
ures in a dance of merry children, come and go with the 
ebb and flow of the annual tide of temperature. Each 
springtime the willow-fringed brooksides blush w^ith the 
pulsations of renewed youth. Flowery banks and shadowy 
vistas in the forests reveal cool retreats in summer, when in 
the stillness of the evening we hear the distant mellow song 
of the wood-thrush. The deep, strong, harlequin colours of 
autumn make the island a garden of gorgeous flowers edged 
about by the silvery surf. In winter the babble of the brooks 
is hushed beneath icy coverings, and the bare trees are etch- 
ings on the white pages of the snow. These minor har- 
monies are interwoven all through the melody of the ages. 
Like the white fretwork on the waves of the sea, they ac- 
company the greater changes wrought by unseen agencies. 
We are overpowered by the multitude of questions suggested 
by this great drama of Nature. We long to know what the 
end may be. Why all this beauty and variety in form and 
colour ? Why the scented breeze, the hum of insects, the 
songs of birds, the music of the brooks, the coming and 
going of hills and valleys, and the thousands of other evi- 



THE LIFE HISTORY OF A RIVER 317 

dences of harmoniously working laws ? Was the elevation 
of the land in a far distant time, the crumbling and decay of 
the rocks, the long journeys of the finer fragments down the 
streams, and their deposition in alluvial lands, but to form 
a soil in which violets and lilies might take root and furnish 
nectar for the bees ? We trace the origin and development 
of material things to intangible laws. These at first seem 
but the children of our own brains. We soon learn, how- 
ever, that they are not only external to ourselves, but sway 
and guide us. Man, too, gathers honey from the flowers. 
Such a mighty vision rises before the mind as we watch our 
island passing through its orderly transformations or glance 
upward at the changing constellations above it, that we 
pause, fearing to go farther, lest our fancy lead us astray. 
Our studies have brought us to the threshold of a vast 
temple: to explore it we must grope our way at first and 
laboriously gather facts to guide us in the same manner as 
in attempting to trace the life history of a river. 

We are recalled from dreamland by a new element in the 
scene before us. A canoe, buoyant and graceful, rounds a 
distant headland, traverses the belt of dark water just out- 
side the beating surf, enters one of the sheltered bays, and 
touches the shore. Dark men clad in skins step upon the 
beach. The light canoe is drawn part way out of the water. 
Soon a column of blue smoke rises above the tree-tops, 
spreads inland, and vanishes in the steady blow of the 
breeze from the sea. As time passes, other savages come 
to the island. Villages are built. Fires sweep through the 
forests leaving black ruin in their wakes. The soil is 
stripped of its natural covering, and for a time erosion is 



3l8 RIVERS OF NORTH AMERICA 

accelerated. Large quantities of soil and other rock debris 
are washed down from the hills and encumber the more 
level lands below, destroying for a time their fertility. 
Generations of savages come and go, until a change of as 
great moment to them as was their coming to the animals 
and plants of the island takes place. 

For the first time a sail breaks the even sky-line of the 
sea. A Half-Moon borne proudly on by gentle breezes 
nears the island and enters one of its forest-fringed harbours. 
The changes which follow the coming of civilised man need 
not be dwelt upon. Chief among the events due to the 
greater w^ants of civilised than of savage men, is the removal 
of the forests. The land is cleared of its trees and shrubs. 
Other plants which grew beneath their shelter are extermin- 
ated. Ploughing greatly facilitates the work of the rills and 
rivulets. The precious layer of soil, thin at best, is more 
rapidly removed than formerly, and the sources of its re- 
newal to a great extent destroyed. In time, fields become 
too impoverished to repay cultivation while virgin lands can 
still be had on neighbouring shores for the taking, and are 
abandoned. Destruction follows apace. Gullies and deep 
canyon-like trenches are cut in the sides of the hills, and 
even greater desolation results in the plains below than fol- 
lowed the wild-fire started by the Indians. The island 
loses its archaic loveliness. Its flora is largely laid waste, 
or supplanted by the growth of seeds brought intentionally 
or by accident from other lands. The native birds and 
animals disappear, or, like the plants, are displaced by others 
and in part alien, species. The changes are so profound that 
they are felt not only throughout the fauna and flora, but 



THE LIFE HISTORY OF A RIVER 319 

impress themselves on the topography of the land. No 
longer a source of immediate gain, the island is neglected 
and abandoned. 

The rivers with their increased freight due to the debris 
v^ashed from abandoned fields, progress more rapidly with 
their appointed tasks than before the forests were removed. 
Deltas are formed at the mouth of the streams, the estuaries 
are filled, and in time the waters of the sea are displaced, 
and broad grassy plains due to construction make their ap- 
pearance. The coast-line again becomes a series of sweep- 
ing curves. Tens of thousands of years elapse before the 
last of the conspicuous results due to elevation and subsi- 
dence become obliterated, and during this interval marked 
changes have taken place in the group of monadnocks 
forming the central highland of the island. Some of the 
original consequent streams were larger or had shorter 
courses to the sea than their competitors, and were enabled 
to develop more rapidly. The divides at the heads of 
the more energetic streams receded and new territory is 
added to their hydrographic basins. This process of cap- 
ture and diversion leads to still greater diversity in the 
topography. The struggle between streams for the posses- 
sion of territory in progress along every divide is not unlike 
the struggle for existence and the survival of the fittest in 
the animal world. The streams develop in accordance with 
their environment. Those most favoured capture the 
waters of their less favoured neighbours and wax stronger 
at the expense of the weak. 

In its old age our island loses the roughness of surface 
produced by stream corrasion. The ridges and peaks be- 



320 RIVERS OF NORTH AMERICA 

come subdued and their outlines more flowing, and the 
valleys broader. In time low mounds alone remain to mark 
the site of once picturesque peaks, and in the broad valleys 
sluggish streams meandering in sweeping curves carry off 
the decreased water supply. Even the hills, after a pro- 
longed old age, disappear, and an undulating plain but 
slightly elevated above the encircling sea remains. A 
geographical cycle has run its course. The resulting 
peneplain has even less diversity than the surface of the 
new-born island. The streams, now flowing sluggishly on 
account of the lowering of their gradients, are too feeble to 
carry burdens, and run clear, but their chemical activity is 
undiminished, and they still bear invisible loads in solution. 
Chemical degradation, previously of minor importance in 
reference to the work of mechanical agencies, now becomes 
the more potent, and the final reduction of the land to sea- 
level is secured by the removal of its material in solution. 

The waves and currents of the sea have been active 
throughout this long history in producing changes which, 
however, are beyond the limits of the present discussion. 

After the streams ceased to bring debris to the shore, 
which, we may presume, either in part or wholly counter- 
acted the attacks of the sea on the land, the low coastal 
plains are washed away, and finally the waves roll over the 
site of the vanished island. 



INDEX 



yEolian corrasion, mention of, 2g 
Ages of terraces, relative, 170, 171 
Aggrading, explanation of the term, 

98 

Alaska, characteristics of the rivers 

of, 284-289 
Alluvial cones, description of, 101- 

109 

— fans, reference to, loi 

— rivers, characteristics of, 264, 265 
Alsec River, Alaska, mention of, 284 
Analyses of river-water, average, 80 

— table of, 78 
Analysis of rain-water, 75 

Anchor ice, influence of, on stream 
transportation, 25-28 

Anticlinals, influence of, on topo- 
graphy, 198 

Appalachian Mountains, stream ad- 
justment in, 195-203 

— rivers, brief account of, 260, 261 
Arctic drainage slope briefly defined, 

256, 257 
Atlantic drainage slope briefly de- 
fined, 256 

Babb, C. C, observations by, 74 

Baselevel, definition of, 47 

Baselevelling, discussion of, 46-50 

Bear River, Wyoming, analysis of the 
water of, 78 

Beheaded streams, explanation of the 
term, 191 

Bering drainage slope briefly defined, 
257 

Big Bend of Columbia River, Wash- 
ington, mention of, 280 



Big Wills Creek, Alabama, adjustment 
of, 208-213 

Bischof, G., cited on water analyses, 79 

Blatchley, W. S., reference to writ- 
ings of, 96 

Bonneville, Lake, reference to deltas 
of, 125 

Bottom loads of streams, 68-70 

— terraces, origin and nature of, 166, 

167 

Branner, J. C, reference to work of, 

X., II 

Breccia, due to faulting, mention of, 4 

Calcium carbonate, solubility of, 92 
Call, R. E., reference to writings of, 

94 
Campbell, M. R., reference to work 

of, X. 
Canada, characteristics of the rivers 

of, 290-292 
Canadian Geological Survey, reference 

to, xi. 
Canyon of Snake River, Washington, 

reference to, 160 

— rivers, characteristics of, 271-275 
Caribbean drainage slope briefly de- 
fined, 257 

Carrollton, Mississippi, sediment in 
the Mississippi at, 71, 72 

Cascade Mountains, reference to 
waterfalls of, 57, 61 

— streams of, 280 

Catskill Mountains, migration of di- 
vides on, 251-253 

Cephalonia, Greece, reference to 
"sea-mills" of, 95 



321 



3^ 



INDEX 



Chamber! in, T. C, reference to the 

work of, X. 
Chandler, C. F., water analyses by, 

Characteristics of American rivers, 

254-299 
Chattanooga, Tenn., geography near, 

208-214 
Chelan, Lake, Washington, terraces 

near, 182 
Chemical degradation, 82-84 

— denudation, discussion of, 80, 81 

— disintegration, discussion of, 6-1 1 
Chesapeake Bay, map of, 219 
Chipaway River, reference to, 138 
Clarke, F. W., water analyses by, 78 
Climate, influence of, on streams, 140- 

142 
Climatic changes, influence of, on 
streams, 223-233 

— on terrace-making, 158-160 
Colorado River, characteristics of, 

271-275 

— reference to, 133 

— still currading, 45 

Columbia River, characteristics of, 
278-282 - - 

— terraces of, 180-182 
Corrasion, discussion of, 28-36 

— lateral, discussion of, 34-36 

— stream, genera] process of, 142- 

145 ^ 
Corthell, E. L., reference to writings 

of, 132 
Crevasses, origin and nature of, 120 
Crosby, F. W., reference to writings 

of, 95 

— W. O., reference to writings of, 

95 
Croton River, New York, analysis of 
the water of, 78 

— material carried in suspension by, 

Cumberland River, Tennessee, analy- 
sis of the water of, 78 

Current terraces, origin and nature of, 
167-169 

Curves made by streams, 36-39 

Danube River, data concerning, 74, 

75 

— material carried in suspension by, 

79 



Darton, N. H., cited on drainage of 
Catskill Mountains, 251 

— reference to the work of x. 
Davis, W. M., cited on lakes, 122 

— cited on peneplains, 48 

— cited on stream development, 187 

— references to the writings of, x., 41, 

42, 214 

— and J. VV. Wood, cited on super- 

imposed drainage, 243 

Decay of rocks, i-ii 

Deflection of streams owing to the 
earth's rotation, 39-43 

Degradation of the land, general rate 
of, 81-84 

Delaware River, analysis of the water 
of, 78 

Delta of the Yukon River, brief ac- 
count of, 288 

— terraces, origin and nature of, 

167-169 
Deltas, origin and structure of, 123- 

142 
Deposition, stream, general process 

of, 142-145 
Deposits made by streams, variations 

in the, 136-142 
Development of streams, 63-66 
Diller, J. S., reference to the work 

of, X. 
Discharge of the Mississippi, 267 
Disintegration of rocks, i-ii 
Distributaries, explanation of the 

term, 103 
Diverted streams, explanation of the 

term, 192 
Divides, migration of, 247-253 
Dodge, R. E., cited on terraces, 157, 

158 

Drainage slopes of North America 
briefly defined, 256, 257 

Drew, F., reference to the writings 
of, lOI 

Drift-wood, influence of, on stream 
development, 240-244 

Drowned rivers, examples of, 260 

Dunes, influence of, on streams, 

139 
Dutton, C. E., reference to explora- 
tions by, X. 

Earth's rotation, influence of, on 
streams, 39-43 



INDEX 



323 



Elevation, effects of, on stream devel- 
opment, 215-217 
England, rate of land degradation in, 

Erosion, baselevel of, discussed, 46- 

50 

— general discussion of, 46-51 

"Fall line" of the Atlantic coast, 
brief account of, 261 

Fault breccia, mention of, 4 

Fergusson, J., reference to the writ- 
ings of, 37 

Ferrel, \V., cited on the rotation of 
the earth, 41 

Flood-plains, origin and nature of, 
1 10-116 

Floods in rivers, brief account of, 
229-233 

Fluctuations of streams, discussion of, 
229-233 

Forshey, Professor, observations by, 
70 

Eraser River, British Columbia, char- 
acteristics of, 282-284 

Ganges River, data concerning, 75 
Geikie, A., references to the writings 

of, 18, 74 
Genesee River, New York, analysis of 

the water of, 78 
Geographical cycles, definition of, 49 
Gilbert, G. K., cited on Niagara 

Falls, 59, 60 

— explorations by, x. 

— reference to writings of, 31, 42, 

125 
Glacial corrasiou, mention of, 29 

— meal, contnbution of, to streams, 

— terraces, origin and nature of, 169, 

170 

Glaciated lands, rivers of, 262, 263 

Glaciers, influence of, on stream de- 
velopment, 234-236 

Grand Coulee, Washington, mention 
of, 280 

Great Basin, climatic condition of, 
17S, 179 

— drainage, brief account of, 257 
Great Falls, Canada, mention of, 58 
Great Lakes, rivers flowing to, 292- 

3QO 



Great Plateaus, references to, 44, 45 
Green River, Kentucky, references 

to, 88-90, 94 
Ground ice, influence of, on stream 

transportation, 25-28 
Gulf drainage slope briefly defined, 

257 

Hayes, C. W., cited on geography of 
Southern Appalachians, 208-214 

— references to the work of, x., 207, 

208 
Hicks, L. E., cited on flood-plains, 
118 

— cited on profiles of streams, 146, 

150 
High Plateaus, reference to, 45 
Hitchcock, E., cited on delta terraces, 

167 
Hoang Ho River, data concerning, 

75 

Holmes, W. H., cited on Colorado 
River, 273 

Hosford, E. N., water analvsis by, 
78 

Hovey, H. C, references to the writ- 
ings of, 94, 96 

Pludson Bay drainage slope briefly ) 
defined, 256 j 

Hudson River, New York, analysis of ' 
the water of, 78 

— brief account of, 260 

— cited as an example of a drowned 

river, 218 — 

— material carried in suspension by, 

79 <-~ 

Humboldt River, Nevada, analysis of 

the water of, 78 
Humphreys and Abbot, cited in ref- 
erence to the Mississippi, 70, 73, 
253, 267 ; cited on drift-wood, 

.243 

— cited on Mississippi delta, 132 
Hunt, T. S., water analysis by, 78 
Hydration, influence of, on rock dis- 
integration, 6 

Ice, influence of, on stream transport- 
ation, 22-28 

— weight of, 23 

Indian Creek, California, reference to, 

I II 
Invisible load of streams, 75-81 



324 



INDEX 



Irrawaddy River, data concerning, 74 

James River, Virginia, analysis of the 
water of, 78 

Jones, W. J., water analysis by, 78 

Jordan River, Utah, analysis of the 
water of, 78 

Jukes-Browne, A. J., reference to the 
writings of, 18 

Julian, A. A., references to the writ- 
ings of, II, 76 

Keyes, C. R., cited on the meander- 

ings of streams, 113 
Kittatinny peneplain, brief account 

of, 200, 206 
Kowak River, Alaska, mention of, 

286 

Lake Pepin, Wisconsin- Minnesota, 
reference to, 138 

— St. Clair, delta in, 133 

— Tahoe, California-Nevada, analy- 

sis of the water of, 78 
Lakes, climatic changes indicated by, 

220 
Laurentian Basin, rivers of, 292-300 
Le Conte, J., references to the writ- 
ings of, 18, 19 
Levees, natural, origin and nature of, 

I 16-123 
Life history of a river, 301-320 
Limestone, solubility of, 92 
Loads of streams, how obtained, 13-16 
Loew, O., water analysis by, 78 
London, England, analysis of rain- 
water at, 75 
Lookout Mountain, Tennessee-Ala- 
bama, drainage of, 208-214 
Los Angeles River, California, analy- 
sis of the water of, 78 
Lost rivers, reference to, 226 
Lupton, N. T., water analysis by, 78 
Luray Cavern, Virginia, references 

to, 93, 94 
Lyell, C, reference to the writings 
of, 22 

Mackenzie River, delta of, 133 
Maine, coast topography of, 218 
Mammoth Cave, Kentucky, brief ac- 
count of, 88, 89, 91, 94 



Marsh, G. P., reference to the writings 
of, II 

Mason, W. P., references to the writ- 
ings of, 75, 76 

Matapediac River, New Brunswick, 
anchor ice in, 25-27 

Material in suspension, measures of, 

70-75 

Maumee River, Ohio, analysis of the 
water of, 78 

Maxwell, W., reference to the writ- 
ings of, II 

McGee, W. J., reference to the work 
of, X. 

Meandering streams, discussion of, 

36-3S 

Mechanical disintegration of rocks, 

2-6 
Merrill, G. P., cited on hydration, 6 

— references to the writings of, 9, 11 
Migration of divides, discussion of, 

247-253 

— of waterfalls, discussion of, 60-63 
Mississippi River, analysis of the 

water of, 78 

— characteristics of, 265-271 

— Commission, reference to map by, 

122 

— data concerning, 74, 75 

— delta of, 131 

— influence of earth's rotation on, 42 

— inundation of, 119-121 

— material carried in suspension by, 

— rate of degradation in basin of, 

83, 84 

— sediment in waters of, 70-74 
Missouri River, an aggrading stream, 

43, 44 
Mohawk River, New York, analysis 

of the water of, 78 
Monadnock, definition of, 49 

— Mount, New Hampshire, refer- 

ence to, 49 
Montmorenci Falls, Canada, reference 

to, 58 
Morrill, P., cited on the Mississippi, 

267 

— reference to the writings of, 121, 

253 
Moses Lake, Washington, reference 

to, 139 
Moulins, mention of, 34 



INDEX 



325 



Murray, J., cited on water analyses, 

So 

Natural Bridge, Virginia, reference 
to, 94 

— levees, origin and nature of, 116- 

123 
Newberry, J. S., explorations by, x. 
New England rivers, characteristics 

of, 259, 260 
New Orleans, Louisiana, depth of 

delta deposits at, 132 
Niagara Falls, profile and section at, 

60 

— reference to, 62 

Niagara River, characteristics of, 296- 

'298 
Nile River, data concerning, 74, 79 
Nita crevasse, Louisiana, an account 
of the, 121, 122 

Ottawa River, (Canada, analysis of the 
water of, 78 

— mention of, 292 

Pacific drainage slope, brief account 
of, 257 

Passaic River, New Jersey, analysis of 
the water of, 78 

Peneplain, definition of, 48 

Peneplains, ancient, in the Appalach- 
ians, 205-207 

Platte River, Nebraska, an aggrading 
stream, 43, 44 

Po River, Italy, data concerning, 74, 

Porcupine River, Alaska, ice-work on 

banks of, 24 
Pot-holes, origin and nature of, 33, 34 
Potomac River, Virginia, data con- 
cerning, 74 

— rate of degradation by, 82 
Powell, J. W., cited on baselevel, 47 

— cited on moisture necessary for 

vegetation, 237 

— cited on rapids in Colorado River, 

138 

— reference to explorations by, x. 
Precipitation, influence of variations 

in, on streams, 224-228 
Profiles of streams, 145-151 

Rain-water, impurities in, 75, 76 



Reade, T. M., cited on chemical de- 
gradation, 82 

Red River, Louisiana, lakes on the 
sides of, 122 

Regolith, meaning of the term, 9 

Reversed streams, explanation of, 
192 

Rhine River, material carried in sus- 
pension by, 79 

Rhone River, data concerning, 74, 

75, 79 

Rio Grande, data concerning, 74 

Rio Grande del Norte, analysis of the 
water of, 78 

River piracy, discussion of, 203-205 

Rocky Mountains, reference to water- 
falls of, 57 

Rotation of the earth, influence of, on 
streams, 39-43 

Roth, Professor, cited in water analy- 
sis, 79 

Russell, L C., cited on glaciers, 236 

— cited on Lauren tian basin, 293 

— cited on Laurentian lakes, 294 

— cited on terraces of the Columbia, 

180-182 

— references to the writings of, 11, 

24, 123, 124, 134, 137, 174 

— T., reference to the writings of, 

of, 253 

Sacramento River, California, analysis 
of the water of, 78 

St. Anthony P'alls, Minnesota, refer- 
ence to, 62 

S't. Clair Lake, delta in, 133 

St. Lawrence drainage slope briefly 
described, 256 

St. Lawrence River, analysis of the 
water of, 78 

— character of, 43 

— submerged portion of, 218 
Salisbury, R. D., reference to the 

writings of, x., 170 
Schooley peneplain, reference to, 205 
Screes, explanation of the term, 109 
Shaler, N. S., cited on caverns, go 
Shenandoah peneplain, brief account 

of, 200 
Shoshone Falls, Idaho, reference to, 

62 
Sierra Nevada, reference to water- 
falls of, 57 



326 



INDEX 



Sierra Nevada rivers, characteristics 
of, 275-278 

Sink-holes, reference to, 93 

Snake River, Idaho-Washington, char- 
acteristics of, 27g, 280 

Snickers Gap, Virginia, character and 
origin of, 200, 201 

Southern rivers, characteristics of, 
263, 264 

Stevenson, D., reference to the writ- 
ings of, 18 

Stickine River, Alaska-Canada, men- 
tion of, 284 

Stream conquest, discussion of, 203- 
205 

— deposition, discussion of, 97-151 

— development, discussion of, 184- 

195 

Subimposed drainage, term suggested, 
246 

Subsequent streams, origin and nature 
of, 184, 185 

Subsidence, effects of, on stream de- 
velopment, 217-221 

Superimposed drainage, explanation 
of, 244-246 

Synclinal mountains and anticlinal 
valleys, 207-214 

Synclinals, influence of, on topo- 
graphy, 198 

Tahoe Lake, California-Nevada, an- 
alysis of the water of, 78 

Taku River, Alaska, mention of, 284 

Talus slopes, origin and nature of, 
109, no 

Tarr, R. S., cited on drowned rivers, 
219 

— cited on young valleys, 55 

— reference to the work of, xi. 
Teanaway River, Washington, dam 

of drift-wood on, 243 
Temperature, influence of variation 

in, on streams, 228, 229 
Terraces, origin and nature of, 152- 

183 

Thames River, England, material 
carried in suspension by, 79 

Thompson, W. C, cited on anchor 
ice, 25-27 

Tides in Columbia River, 280 

Todd, J. E., cited on the Mississippi, 
270 



Transportation by streams, discussion 
of, 14-28 

Trenton Falls, New York, reference 
to, 58 

Troy, New York, impurities in rain- 
water at, 76 

Truckee River, Nevada, analysis of 
the water of, 78 

Tundra of Arctic shores, brief account 
of, 133, 288 

Underground streams, 84-96 
United States Geological Survey, re- 
ference to work of, xi. 
Uruguay River^ data concerning, 74 

Vegetation, influence of, on stream 
development, 236-244 

Visible loads of streams, 67-75 

Volcanic agencies, influence of, on 
stream development, 231-233 

Volcanic dust, contributed to streams, 
14 

Von Hosen, J., table of analyses com- 
piled by, 78 

Wales, rate of land degradation in, 

83 

Walker River, Nevada, analysis of 
the water of, 78 

Walla Walla River, Washington, re- 
ference to, 137 

Waller, E., water analysis by, 78 

Water, weight of, 23 

— analyses, table of, 78 
Waterfalls, nature and history of, 

54-62 

Water-gaps, origin of, 199-205 

Watkins Glen, New York, reference 
to, 58 

Wheeler, W. H., reference to the 
writings of, 73 

White River, Washingson, mention 
of, 287 

Wilbur, E. M., cited on tides in Co- 
lumbia River, 2S0 

Willis, B., cited on stream adjust- 
ment, 198 

— reference to the work of, xi. 

— reference to the writings of, 200 
Wills Creek, Alabama, adjustment of, 

208-213 



INDEX 



327 



Wind-gaps, explanation of the term, 

199-205 
Wurtz, H., water analysis by, 78 
Wyandotte Cavern, Indiana, refer- 
ence to, 93 



Yazoo River, Louisiana, reference to, 

123 
Young valleys, illustrations of, 55 
Yukon River, ice-work on the banks 

of, 24 
— drift-wood on, 242 



The Science Series 



Edited by Professor J. McKeen Cattell, Columbia Uni- 
versity, with the cooperation of Frank Evers Beddard, 
F.R.S., in Great Britain. 

Each volume of the series will treat some department of 
science with reference to the most recent advances, and will 
be contributed by an author of acknowledged authority. 
Every effort will be made to maintain the standard set by the 
first volumes, until the series shall represent the more im- 
portant aspects of contemporary science. The advance of 
science has been so rapid, and its place in modern life has 
become so dominant, that it is needful to revise continually 
the statement of its results, and to put these in a form that is 
intelligible and attractive. The man of science can himself 
be a specialist in one department only, yet it is necessary for 
him to keep abreast of scientific progress in many directions. 
The results of modern science are of use in nearly every pro- 
fession and calling, and are an essential part of modern 
edacation and culture. A series of scientific books, such as 
has been planned, should be assured of a wide circulation, 
and should contribute greatly to the advance and diffusion of 
scientific knowledge. 

The volumes will be in octavo form, and will be fully illus- 
trated in so far as the subject-matter calls for illustrations. 



G. P. PUTNAM'S SONS, New York & London 



THE SCIENCE SERIES 



(Volumes ready, in press, and in preparation.) 
The Study of Man. By Professor A. C. Haddon, M.A., D.Sc, Royal 
College of Science, Dublin. Illustrated. 

The Groundwork of Science. A Study of Epistemology. By St. 
George Mivart, F.R.S. 

Rivers of North America. A Reading Lesson for Students of Geography 
and Geology. By Israel C. Russell, LL.D., Professor of Geology 
in the University of Michigan. Illustrated. 

Earth Sculpture. By Professor James Geikie, F.R.S., University of 
Edinburgh. Illustrated, 

The Stars. By Professor Simon Newcomb, U.S.N., Nautical Almanac 

Office, and Johns Hopkins University. 
Meteors and Comets. By Professor C. A. Young, Princeton University. 
The Measurement of the Earth. By Professor T. C. Mendenhall, 

Worcester Polytechnic Institute, formerly Superintendent of the U. S. 

Coast and Geodetic Survey. 

Volcanoes. By T. G. Bonne y, F.R.S., University College, London. 

Earthquakes. By Major C. E. Button, U.S.A. 

Physiography; The Forms of the Land. By Professor W. M. Davis, 

Harvard University. 
The History of Science. By C. S. Peirce. 
General Ethnography. By Professor Daniel G. Brinton, University 

of Pennsylvania. 
Recent Theories of Evolution. By J. Mark Baldwin, Princeton 

University. 
Whales. By F. E. Beddard, F.R.S., Zoological Society, London. 
The Reproduction of Living Beings. By Professor Marcus Hartog, 

Queen's College, Cork. 
Man and the Higher Apes. By Dr. A. Keith, F.R.C.S. 
Heredity. By J. Arthur Thompson, School of Medicine, Edinburgh. 
Life Areas of North America : A Study in the Distribution of 

Animals and Plants. By Dr. C. Hart Merriam, Chief of the Bio- 
logical Survey, U. S. Department of Agriculture. 
Age, Growth, Sex, and Death. By Professor Charles S. Minot, 

Harvard Medical School. 
Bacteria. Dr. J. H. Gladstone. 
History of Botany. Professor A. H. Green. 
Planetary Motion. G. W. Hill. 
Infection and Immunity. Geo. M. Sternberg, Surgeon-General U.S.A. 



G. P. PUTNAM'S SONS, New York & London 



* 






V * o . ^ 



V/. 






0- X • *^ o^ 






^. 


v\ 












t/' 


^•^ 












'/• 


^" 








"^^ 


V 


aV^' 


">. 


<" 


^/. 






\ 




\ ; 






o 






^ . r 




.^^^^ 


y. 


-.^'^ 




^ ^ 

% 


..x^'' 


■/ 

^ 




'" " / 




.4* 



xXV-^. 



■.,,.* .A 






i\ ■ .-.'■■■♦ • o 



.-X' 



,\ 



0' 



>" 









\"' -^^ 






V/ 



i: 






'■Jo « ;^'-' r.--' -• - '^ 






-/. ,,N 






.-^'^ 



\ 












,\ 



- . . ,, ,/:. ^-"0^ 

'"'^V'^^-/^ 



>^ -^^fr?^ 



o. / , 






>^ xN^ 



-'^:r. ^^■ 



s- ^. 



A^ 









^^A^ 



'■> ". 






'^'% 



>" .v'«. ^/^ ^'^ ^ ^ 0.\^ 



.^^\^.^ 



.^^ 



,0 -/ 






.^*\ 



■\^^ 






.^ .x-' ■■..,-. ■• -(-.. ,-ji 



j-- <^ 



V 'N^ 



'^^- >^^ 



.-,-^ •'■r: 



%.<^ 



,\' 


^> 




o 




"♦ •/ 



nX^^' 



''^.. 



■^•';^' 



•^^^^ 



. ^<> 



,0' 









■<>;^' 



^ ' '^t' .-A 



v^'' 






■•'■ O^ 



^. 



S •'', 



^•'•0^ 















-^^ A- 






•'^^<=.'^ 



■\<^'' 






V V^ 



r:)-^vi^^^WKfrny)jm.yiisf^^^\K"' • ■-- ^. 










OF 



CONGRESS 



708 3247 









l!«Wt« 






'?v>''>''i^'§;-:fS!iii:; 



mmm'.. 






wm. 



irm'E'ji'-it 



-?i;!iiSI''' 



